xfs: indicate if xfs_bmap_adjacent changed ap->blkno

Add a return value to xfs_bmap_adjacent to indicate if it did change
ap->blkno or not.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>