bgpd: ignore NEXT_HOP for MP_REACH_NLRI

bgpd: ignore NEXT_HOP for MP_REACH_NLRI

RFC 4760 states we SHOULD ignore the NEXT_HOP attribute for BGP Update
messages carrying only MP_REACH_NLRI attributes. Thus we should use the
Network Address of Next Hop field of the MP_REACH_NLRI as the nexthop.

Instead of always looking for BGP_ATTR_NEXT_HOP, this commit ensures:
1) we set mp_nexthop_len to BGP_ATTR_NHLEN_IPV4 for v4 bgp_static routes
2) we check mp_nexthop_len when choosing the nexthop to use for nht
3) we check mp_nexthop_len when choosing the nexthop to send to zebra
4) we check mp_nexthop_len when picking the nexthop to shown by vtysh

Reported-by: Binon Gorbutt <binon@aervivo.com>
Signed-off-by: Trey Aspelund <taspelund@nvidia.com>