congestion_wait(): don't use WRITE

commit 8aa7e847 (Fix congestion_wait() sync/async vs read/write
confusion) replace WRITE with BLK_RW_ASYNC.  Unfortunately, concurrent mm
development made the unchanged place accidentally.

This patch fixes it too.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>