vxlan: advertise link netns in fdb messages

Previous commit is based on a wrong assumption, fdb messages are always sent
into the netns where the interface stands (see vxlan_fdb_notify()).

These fdb messages doesn't embed the rtnl attribute IFLA_LINK_NETNSID, thus we
need to add it (useful to interpret NDA_IFINDEX or NDA_DST for example).

Note also that vxlan_nlmsg_size() was not updated.

Fixes: 193523bf9373 ("vxlan: advertise netns of vxlan dev in fdb msg")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>