suspend: simplify block I/O handling

commit 343df3c7 upstream.

Stop abusing struct page functionality and the swap end_io handler, and
instead add a modified version of the blk-lib.c bio_batch helpers.

Also move the block I/O code into swap.c as they are directly tied into
each other.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Ming Lin <mlin@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Rafael J. Wysocki <rjw@rjwysocki.net>
Signed-off-by: Jens Axboe <axboe@fb.com>
[bwh: Backported to 3.16 as dependency of commit f6cf0545
 "PM / Hibernate: Call flush_icache_range() on pages restored in-place":
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>