suspend: simplify block I/O handling

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>

diff --cc include/linux/swap.h
Simple merge

diff --cc kernel/power/Makefile
Simple merge

diff --cc kernel/power/block_io.c
Simple merge

diff --cc kernel/power/power.h
Simple merge

diff --cc kernel/power/swap.c
Simple merge

diff --cc mm/page_io.c
Simple merge