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: NMarkus Trippelsdorf <markus@trippelsdorf.de>
Reported-by: NMiles Lane <miles.lane@gmail.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -2306,10 +2306,8 @@ void ip_mc_drop_socket(struct sock *sk)
 
 		in_dev = inetdev_by_index(net, iml->multi.imr_ifindex);
 		(void) ip_mc_leave_src(sk, iml, in_dev);
-		if (in_dev != NULL) {
+		if (in_dev != NULL)
 			ip_mc_dec_group(in_dev, iml->multi.imr_multiaddr.s_addr);
-			in_dev_put(in_dev);
-		}
 		/* decrease mem now to avoid the memleak warning */
 		atomic_sub(sizeof(*iml), &sk->sk_omem_alloc);
 		call_rcu(&iml->rcu, ip_mc_socklist_reclaim);