Require INVARIANTS and WITNESS if DEBUG_VFS_LOCKS is set

Reported by:	pho
Reviewed by:	markj, mjg
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D38070

sys/kern/vfs_subr.c

@@ -99,6 +99,10 @@ __FBSDID("$FreeBSD$");
 #include <vm/vm_kern.h>
 #include <vm/uma.h>
 
+#if defined(DEBUG_VFS_LOCKS) && (!defined(INVARIANTS) || !defined(WITNESS))
+#error DEBUG_VFS_LOCKS requires INVARIANTS and WITNESS
+#endif
+
 #ifdef DDB
 #include <ddb/ddb.h>
 #endif