xfs: xfs_kset should be static

As it is accessed through the struct xfs_mount and can be set up
entirely from fs/xfs/xfs_super.c

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>