xfs: also reject BULKSTAT_SINGLE in a mount user namespace

BULKSTAT_SINGLE exposed the ondisk uids/gids just like bulkstat, and can
be called on any inode, including ones not visible in the current mount.

Fixes: f736d93d

 ("xfs: support idmapped mounts")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>