xfs: fall back to native ioctls for unhandled compat ones

Always try the native ioctl if we don't have a compat handler.  This
removes a lot of boilerplate code as 'modern' ioctls should generally
be compat clean, and fixes the missing entries for the recently added
FS_IOC_GETFSLABEL/FS_IOC_SETFSLABEL ioctls.

Fixes: f7664b31 ("xfs: implement online get/set fs label")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>