xfs: add missing nrext64 inode flag check to scrub

Add this missing check that the superblock nrext64 flag is set if the
inode flag is set.

Fixes: 9b7d16e3

 ("xfs: Introduce XFS_DIFLAG2_NREXT64 and associated helpers")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>