inet: fix ip_mc_drop_socket()

commit 8723e1b4 (inet: RCU changes in inetdev_by_index())
forgot one call site in ip_mc_drop_socket()

We should not decrease idev refcount after inetdev_by_index() call,
since refcount is not increased anymore.

Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Reported-by: Miles Lane <miles.lane@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>