The callers of kern_getfsstat(UIO_SYSSPACE) expect that *buf always

returns memory which must be freed, regardless of the error.  Assign
NULL to *buf in case we are not going to allocate any memory due to
invalid mode.

Reported and tested by:	pho
Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks (together with r310638)
Differential revision:	https://reviews.freebsd.org/D9042

sys/kern/vfs_syscalls.c

@@ -429,6 +429,8 @@ kern_getfsstat(struct thread *td, struct statfs **buf, size_t bufsize,
 	case MNT_NOWAIT:
 		break;
 	default:
+		if (bufseg == UIO_SYSSPACE)
+			*buf = NULL;
 		return (EINVAL);
 	}
 restart: