mptcp: mpc: check fallback to TCP in case of drop

After 3 failed attempts, there is a fallback to TCP.

Link: multipath-tcp/mptcp_net-next#477
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

gtests/net/mptcp/mp_capable/v1_mp_capable_connect_drop_fallback.pkt

@@ -0,0 +1,27 @@
+--tolerance_usecs=100000
+`../common/defaults.sh`
+
+0.0     socket(..., SOCK_STREAM, IPPROTO_MPTCP) = 3
++0.0    setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
++0.0    getsockopt(3, SOL_TCP, TCP_IS_MPTCP, [1], [4]) = 0
++0.0    fcntl(3, F_GETFL) = 0x2 (flags O_RDWR)
++0.0    fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
+
+// Establish connection and verify that there is a fallback to TCP.
+
++0     `nstat -n`
++0.0    connect(3, ..., ...) = -1  EINPROGRESS (Operation now in progress)
++0.0      >  S   0:0(0)                    <mss 1460, sackOK, TS val 100 ecr 0,   nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
++1.0      >  S   0:0(0)                    <mss 1460, sackOK, TS val 100 ecr 0,   nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
++1.0      >  S   0:0(0)                    <mss 1460, sackOK, TS val 100 ecr 0,   nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
+
++1.0      >  S   0:0(0)                    <mss 1460, sackOK, TS val 100 ecr 0,   nop, wscale 8>
++0.01     <  S.  0:0(0)  ack 1  win 65535  <mss 1460, sackOK, TS val 700 ecr 100, nop, wscale 8>
++0.0      >   .  1:1(0)  ack 1             <nop, nop,         TS val 100 ecr 700>
+
++0.200  getsockopt(3, SOL_SOCKET, SO_ERROR, [0], [4]) = 0
+
+// Check for fallback
++0.0    getsockopt(3, SOL_TCP, TCP_IS_MPTCP, [0], [4]) = 0
++0.01  `test $(nstat -zjs | jq '.kernel.MPTcpExtMPCapableSYNTXDrop') -eq 1`
++0.01  `test $(nstat -zjs | jq '.kernel.MPTcpExtBlackhole') -eq 1`