block: use rq complete marking in blk_abort_request()

We cannot abort a request if we raced with the timeout handler already,
or with the IO completion. So make blk_abort_request() mark the request
as complete, and only continue if we succeeded.

Found and suggested by Mike Anderson <andmike@linux.vnet.ibm.com>

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>