dm raid: fix compat_features validation

dm raid: fix compat_features validation

In ecbfb9f118bce4 ("dm raid: add raid level takeover support") a new
compatible feature flag was added.  Validation for these compat_features
was added but this only passes for new raid mappings with this feature
flag.  This causes previously created raid mappings to be failed at
import.

Check compat_features for the only valid combination.

Fixes: ecbfb9f118bce4 ("dm raid: add raid level takeover support")
Cc: stable@vger.kernel.org # v4.8
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>