xfs: add missing assert in xfs_fsmap_owner_from_rmap

commit 110f09cb705af8c53f2a457baf771d2935ed62d4 upstream.

The fsmap handler shouldn't fail silently if the rmap code ever feeds it
a special owner number that isn't known to the fsmap handler.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>