vfs: Return early from sysctl_vfs_ctl() if no input was given

Otherwise we end up searching for a mountpoint using an uninitialized
key, and likely failing the version test.  This violates KMSAN's
invariants, so simply return immediately instead.

MFC after:	2 weeks

sys/kern/vfs_subr.c

@@ -6519,6 +6519,8 @@ sysctl_vfs_ctl(SYSCTL_HANDLER_ARGS)
 	int error;
 	struct mount *mp;
 
+	if (req->newptr == NULL)
+		return (EINVAL);
 	error = SYSCTL_IN(req, &vc, sizeof(vc));
 	if (error)
 		return (error);