mpage: split submit_bio and bio end_io handler for reads and writes

mpage: split submit_bio and bio end_io handler for reads and writes

Split the submit_bio() and bio end_io handler for reads and writes similar
to other aops.

This is a prep patch before we convert end_io handlers to use folios.

Link: https://lkml.kernel.org/r/20230411122920.30134-3-p.raghav@samsung.com
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Martin Brandenburg <martin@omnibond.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Marshall <hubcap@omnibond.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>