net: Fix dev_mc_add()

Commit 6e17d45a (net: add addr len check to dev_mc_add)
added a bug in dev_mc_add(), since it can now exit with a lock
imbalance.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>