Fix rdmap message to be splited into two segments on receiver

【BUG DESCRIPTION】
Testcase `ib_write_lat -d urdma_0 -i 1 -x 0 -R -n 5 -s 976` will hang if
run with 82599 NIC.

On the receiver, `ddp_place_tagged_data` can only handle one mbuf
segment, but the rdmap message will be splited into two segments by
the 82599 port.

Dump tx mbuf on sender:

```txt
USER1: <dev=0 qp=2> [TX]
dump mbuf at 0x7002e7ecd800, iova=1d9d8cd8c0, buf_len=1654
  pkt_len=1042, ol_flags=f0000000000000, nb_segs=2, in_port=65535
  segment at 0x7002e7ecd800, data=0x7002e7ecd916, data_len=52
  Dump data at [0x7002e7ecd916], len=52
00000000: 70 10 6F AE CD B5 70 10 6F AE CD 9D 08 00 45 00 | p.o...p.o.....E.
00000010: 04 04 00 00 00 00 40 11 00 00 C0 A7 01 03 C0 A7 | ......@.........
00000020: 01 04 F8 44 AF DD 03 F0 87 57 00 00 00 01 00 00 | ...D.....W......
00000030: 00 00 00 00 |  |  |  |  |  |  |  |  |  |  |  |  | ....
  segment at 0x7002e7ecdfc0, data=0x7002e7ecd172, data_len=990
  Dump data at [0x7002e7ecd172], len=990
00000000: C1 40 00 65 49 DD 00 00 00 00 00 69 44 00 00 00 | .@.eI......iD...
00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
.
.
.
000003B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
000003C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
000003D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 01 |  |  | ..............
```

Dump rx mbuf on receiver:

```txt
USER1: <dev=0 qp=2> [RX]
dump mbuf at 0x7002e9004500, iova=1f2d204580, buf_len=1654
  pkt_len=1042, ol_flags=180, nb_segs=2, in_port=0
  segment at 0x7002e9004500, data=0x7002e9004600, data_len=1024
  Dump data at [0x7002e9004600], len=1024
00000000: 70 10 6F AE CD B5 70 10 6F AE CD 9D 08 00 45 00 | p.o...p.o.....E.
00000010: 04 04 00 00 00 00 40 11 F3 93 C0 A7 01 03 C0 A7 | ......@.........
00000020: 01 04 F8 44 AF DD 03 F0 7C A7 00 00 00 01 00 00 | ...D....|.......
00000030: 00 00 00 00 C1 40 00 65 49 DD 00 00 00 00 00 69 | .....@.eI......i
00000040: 44 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | D...............
00000050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
.
.
.
000003D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
000003E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
000003F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
  segment at 0x7002e9004c40, data=0x7002e9004d40, data_len=18
  Dump data at [0x7002e9004d40], len=18
00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 | ................
00000010: 00 01 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | ..
```

【REASON】

The `mbuf_size` will letter than 2K if the MTU is 1500, than the ixgbe PMD
will set the rx buffer to 1KB. Refer to the code below:

```c
ixgbe_dev_rx_init(struct rte_eth_dev *dev)
{
	...
	/*
	 * Configure the RX buffer size in the BSIZEPACKET field of
	 * the SRRCTL register of the queue.
	 * The value is in 1 KB resolution. Valid values can be from
	 * 1 KB to 16 KB.
	 */
	buf_size = (uint16_t)(rte_pktmbuf_data_room_size(rxq->mb_pool) -
		RTE_PKTMBUF_HEADROOM);
	srrctl |= ((buf_size >> IXGBE_SRRCTL_BSIZEPKT_SHIFT) &
		   IXGBE_SRRCTL_BSIZEPKT_MASK);

	IXGBE_WRITE_REG(hw, IXGBE_SRRCTL(rxq->reg_idx), srrctl);
	...
}
```

【FIX】
Refer to the comment of `RTE_MBUF_DEFAULT_BUF_SIZE`, it's the recommended
minimal buffer length.

```c
/**
 * Some NICs need at least 2KB buffer to RX standard Ethernet frame without
 * splitting it into multiple segments.
 * So, for mbufs that planned to be involved into RX/TX, the recommended
 * minimal buffer length is 2KB + RTE_PKTMBUF_HEADROOM.
 */
	(RTE_MBUF_DEFAULT_DATAROOM + RTE_PKTMBUF_HEADROOM)
```

Signed-off-by: ziyang <hh123okbb@gmail.com>

src/urdmad/main.c

@@ -999,6 +999,11 @@ usiw_port_init(struct usiw_port *iface, struct usiw_port_config *port_config)
 	mbuf_size = RTE_PKTMBUF_HEADROOM + port_config->mtu
 		+ ETHER_HDR_LEN + ETHER_CRC_LEN + urdma_vlan_space;
 
+	/* Some NICs (e.g. ixgbe) need at least 2KB buffer to RX standard
+	 * Ethernet frame without splitting it into multiple segments. */
+	mbuf_size = mbuf_size > RTE_MBUF_DEFAULT_BUF_SIZE ?
+		mbuf_size : RTE_MBUF_DEFAULT_BUF_SIZE;
+
 	snprintf(name, RTE_MEMPOOL_NAMESIZE,
 			"port_%u_rx_mempool", iface->portid);
 	RTE_LOG(DEBUG, USER1, "create rx mempool for port %" PRIu16 " with %u mbufs of size %zu\n",