growfs: Commit the changes after expanding the partition

This fix the problem in arm snapshot present since at least 6 months where
growfs was failing at firstboot and dropped you in a single user shell.

etc/rc.d/growfs

@@ -90,6 +90,7 @@ growfs_start ()
 		exit 0
 	}
 }' dev="$rawdev"
+	gpart commit "$rootdev"
 	growfs -y /dev/"$rootdev"
 }