audit: fix signedness bug in audit_log_execve_info()

In the loop, a size_t "len" is used to hold the return value of
audit_log_single_execve_arg(), which returns -1 on error.  In that
case the error handling (len <= 0) will be bypassed since "len" is
unsigned, and the loop continues with (p += len) being wrapped.
Change the type of "len" to signed int to fix the error handling.

	size_t len;
	...
	for (...) {
		len = audit_log_single_execve_arg(...);
		if (len <= 0)
			break;
		p += len;
	}
Signed-off-by: NXi Wang <xi.wang@gmail.com>
Signed-off-by: NEric Paris <eparis@redhat.com>

kernel/auditsc.c

@@ -1362,8 +1362,8 @@ static void audit_log_execve_info(struct audit_context *context,
 				  struct audit_buffer **ab,
 				  struct audit_aux_data_execve *axi)
 {
-	int i;
-	size_t len, len_sent = 0;
+	int i, len;
+	size_t len_sent = 0;
 	const char __user *p;
 	char *buf;