xfs: fix scrub flagging rtinherit even if there is no rt device

The kernel has always allowed directories to have the rtinherit flag
set, even if there is no rt device, so this check is wrong.

Fixes: 80e4e126 ("xfs: scrub inodes")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>