net/ipv6: Correct PIM6 mrt_lock handling

commit 25b4a44c upstream.

In the IPv6 multicast routing code the mrt_lock was not being released
correctly in the MFC iterator, as a result adding or deleting a MIF would
cause a hang because the mrt_lock could not be acquired.

This fix is a copy of the code for the IPv4 case and ensures that the lock
is released correctly.
Signed-off-by: Richard Laing <richard.laing@alliedtelesis.co.nz>
Acked-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

net/ipv6/ip6mr.c

@@ -552,7 +552,7 @@ static void ipmr_mfc_seq_stop(struct seq_file *seq, void *v)
 
 	if (it->cache == &mrt->mfc6_unres_queue)
 		spin_unlock_bh(&mfc_unres_lock);
-	else if (it->cache == mrt->mfc6_cache_array)
+	else if (it->cache == &mrt->mfc6_cache_array[it->ct])
 		read_unlock(&mrt_lock);
 }