rds: add missing barrier to release_refill

The functions clear_bit and set_bit do not imply a memory barrier, thus it
may be possible that the waitqueue_active function (which does not take
any locks) is moved before clear_bit and it could miss a wakeup event.

Fix this bug by adding a memory barrier after clear_bit.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>