xfs: validate v5 feature fields

We don't check that the v4 feature flags taht v5 requires to be set
are actually set anywhere. Do this check when we see that the
filesystem is a v5 filesystem.

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>