xfs: refactor file range validation

Refactor all the open-coded validation of file block ranges into a
single helper, and teach the bmap scrubber to check the ranges.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>