zram: refactor highlevel read and write handling

Instead of having an outer loop in __zram_make_request and then branch out
for reads vs writes for each loop iteration in zram_bvec_rw, split the
main handler into separat zram_bio_read and zram_bio_write handlers that
also include the functionality formerly in zram_bvec_rw.

Link: https://lkml.kernel.org/r/20230411171459.567614-8-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>