vxlan: ecmp support for mac fdb entries

Todays vxlan mac fdb entries can point to multiple remote
ips (rdsts) with the sole purpose of replicating
broadcast-multicast and unknown unicast packets to those remote ips.

E-VPN multihoming [1,2,3] requires bridged vxlan traffic to be
load balanced to remote switches (vteps) belonging to the
same multi-homed ethernet segment (E-VPN multihoming is analogous
to multi-homed LAG implementations, but with the inter-switch
peerlink replaced with a vxlan tunnel). In other words it needs
support for mac ecmp. Furthermore, for faster convergence, E-VPN
multihoming needs the ability to update fdb ecmp nexthops independent
of the fdb entries.

New route nexthop API is perfect for this usecase.
This patch extends the vxlan fdb code to take a nexthop id
pointing to an ecmp nexthop group.

Changes include:
- New NDA_NH_ID attribute for fdbs
- Use the newly added fdb nexthop groups
- makes vxlan rdsts and nexthop handling code mutually
  exclusive
- since this is a new use-case and the requirement is for ecmp
nexthop groups, the fdb add and update path checks that the
nexthop is really an ecmp nexthop group. This check can be relaxed
in the future, if we want to introduce replication fdb nexthop groups
and allow its use in lieu of current rdst lists.
- fdb update requests with nexthop id's only allowed for existing
fdb's that have nexthop id's
- learning will not override an existing fdb entry with nexthop
group
- I have wrapped the switchdev offload code around the presence of
rdst

[1] E-VPN RFC https://tools.ietf.org/html/rfc7432
[2] E-VPN with vxlan https://tools.ietf.org/html/rfc8365
[3] http://vger.kernel.org/lpc_net2018_talks/scaling_bridge_fdb_database_slidesV3.pdf

Includes a null check fix in vxlan_xmit from Nikolay

v2 - Fixed build issue:
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>