swap: fix swapfile read/write offset

commit caf6912f 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...

Fixes: dd6bd0d9

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