libxfs: fix xfs_attr_shortform_bytesfit declaration

Change the xfs_attr_shortform_bytesfit declaration to have
struct xfs_inode to avoid tripping up the libxfs-diff scanner.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>