vxlan: fix dereference of nexthop group in nexthop update path

fix dereference of nexthop group in fdb nexthop group
update validation path.

Fixes: 1274e1cc ("vxlan: ecmp support for mac fdb entries")
Reported-by: Ido Schimmel <idosch@idosch.org>
Suggested-by: Ido Schimmel <idosch@idosch.org>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>