xfs: return FIEMAP_EXTENT_UNKNOWN for delayed allocation extent

For FIEMAP ioctl(2), if an extent is in delayed allocation
state, we need to return the FIEMAP_EXTENT_UNKNOWN flag except
the FIEMAP_EXTENT_DELALLOC because its data location is unknown.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>