xfs: move inode extent size hint validation to libxfs

Extent size hint validation is used by scrub to decide if there's an
error, and it will be used by repair to decide to remove the hint.
Since these use the same validation functions, move them to libxfs.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>