xfs: set XFS_FEAT_NLINK correctly

While xfs_has_nlink() is not used in kernel, it is used in userspace
(e.g. by xfs_db) so we need to set the XFS_FEAT_NLINK flag correctly
in xfs_sb_version_to_features().

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>