block, btrfs, ext4, xfs: Implement swap_rw

Implement swap_rw for block devices, btrfs, ext4 and xfs.  This allows the
the page swapping code to use direct-IO rather than direct bio submission,
whilst skipping the checks going via read/write_iter would entail.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Christoph Hellwig <hch@lst.de>
cc: Jens Axboe <axboe@kernel.dk>
cc: Chris Mason <clm@fb.com>
cc: Josef Bacik <josef@toxicpanda.com>
cc: David Sterba <dsterba@suse.com>
cc: "Theodore Ts'o" <tytso@mit.edu>
cc: Andreas Dilger <adilger.kernel@dilger.ca>
cc: Darrick J. Wong <djwong@kernel.org>
cc: linux-block@vger.kernel.org
cc: linux-btrfs@vger.kernel.org
cc: linux-ext4@vger.kernel.org
cc: linux-xfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org