->b_offset can only be non-zero for _XBF_KMEM backed buffers, so
remove all code dealing with it for page backed buffers.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
[dgc: modified to fit this patchset]
Signed-off-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NDarrick J. Wong <djwong@kernel.org>