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: NChristoph Hellwig <hch@lst.de>
Tested-by: NPavel Machek <pavel@ucw.cz>
Tested-by: NMing Lin <mlin@kernel.org>
Acked-by: NPavel Machek <pavel@ucw.cz>
Acked-by: NRafael J. Wysocki <rjw@rjwysocki.net>
Signed-off-by: NJens Axboe <axboe@fb.com>