zram: make zram_bio_discard more self-contained

Derive the index and offset variables inside the function, and complete
the bio directly in preparation for cleaning up the I/O path.

Link: https://lkml.kernel.org/r/20230411171459.567614-4-hch@lst.de


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