Add a compat handler for XFS_IOC_FSSETDM_BY_HANDLE.

I haven't tested this, lacking dmapi tools to do so
(unless xfsqa magically gets this somehow?)
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>