vxlan: ensure to advertise the right fdb remote

The goal of this patch is to fix rtnelink notification. The main problem was
about notification for fdb entry with more than one remote. Before the patch,
when a remote was added to an existing fdb entry, the kernel advertised the
first remote instead of the added one. Also when a remote was removed from a fdb
entry with several remotes, the deleted remote was not advertised.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>