swim3: clean up request completion paths

swim3 curiously tries to update request parameters before calling
__blk_end_request() when __blk_end_request() will do it anyway, and it
updates request for partial completion manually instead of using
blk_update_request().  Also, it does some spurious checks on rq such
as testing whether rq->sector is negative or current_nr_sectors is
zero right after fetching.

Drop unnecessary stuff and use standard block layer mechanisms.

[ Impact: cleanup ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>