cmd/snap-confine: fix usage of undefined pointer

The debug() line added a few commits ago was attempting to print the
device major/minor from a structure that was only valid inside the scope
of the `for` loop above.

Fix this by printing the device IDs for the device that we are actually
examining.

cmd/snap-confine/ns-support.c

@@ -315,7 +315,7 @@ static bool should_discard_current_ns(dev_t base_snap_dev)
 		}
 	}
 	debug("base snap device %d:%d not found in existing mount ns",
-	      mie->dev_major, mie->dev_minor);
+	      major(base_snap_dev), minor(base_snap_dev));
 	return true;
 }