swap: fix swapfile read/write offset

commit caf6912f3f4af7232340d500a4a2008f81b93f14 upstream.

We're not factoring in the start of the file for where to write and
read the swapfile, which leads to very unfortunate side effects of
writing where we should not be...

[This issue only affects swapfiles on filesystems on top of blockdevs
that implement rw_page ops (brd, zram, btt, pmem), and not on top of any
other block devices, in contrast to the upstream commit fix.]

Fixes: dd6bd0d9 ("swap: use bdev_read_page() / bdev_write_page()")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Cc: stable@vger.kernel.org # 4.9
Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>