read_ns_sys_file(): check if buf is long enough

In case buf size is not adequate, report and return an error.
This is modelled after commit 1ead3d79.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

net.c

@@ -45,6 +45,11 @@ int read_ns_sys_file(char *path, char *buf, int len)
 	rlen = read(fd, buf, len);
 	close(fd);
 
+	if (rlen == len) {
+		pr_err("Too small buffer to read ns sys file %s\n", path);
+		return -1;
+	}
+
 	if (rlen > 0)
 		buf[rlen - 1] = '\0';