net: fix address check in rtnl_fdb_del

Commit 6681712d
	vxlan: generalize forwarding tables

relaxed the address checks in rtnl_fdb_del() to use is_zero_ether_addr().
This allows users to add multicast addresses using the fdb API.  However,
the check in rtnl_fdb_del() still uses a more strict
is_valid_ether_addr() which rejects multicast addresses.  Thus it
is possible to add an fdb that can not be later removed.
Relax the check in rtnl_fdb_del() as well.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>