blk_execute_rq() can oops in wait_for_completion(), if the request has
completed before we call wait_for_completion() because blk_end_sync_rq()
clears ->waiting when it is entered.  Fix this by always using the on-stack
completion variable instead.
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>