block: cleanup the VM accounting in submit_bio

submit_bio uses some extremely convoluted checks and confusing comments
to only account REQ_OP_READ/REQ_OP_WRITE comments.  Just switch to the
plain obvious checks instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220516063654.2782792-1-hch@lst.de


[axboe: fixup WRITE -> REQ_OP_WRITE]
Signed-off-by: Jens Axboe <axboe@kernel.dk>