[PATCH] fix ordering on requeued request drainage

Previously, if a fs request which was being drained failed and got
requeued, blk_do_ordered() didn't allow it to be reissued, which causes
queue stall.  This patch makes blk_do_ordered() use the sequence of each
request to determine whether a request can be issued or not.  This fixes
the bug and simplifies code.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Acked-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>