block: cleanup the memory stall accounting in submit_bio

Instead of a convoluted chain just check for REQ_OP_READ directly,
and keep all the memory stall code together in a single unlikely
branch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>