ipv6: Check the hop limit setting in ancillary data.

When specifing the outgoing hop limit as ancillary data for sendmsg(),
the kernel doesn't check the integer hop limit value as specified in
[RFC-3542] section 6.3.
Signed-off-by: NShan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

net/ipv6/datagram.c

@@ -705,6 +705,11 @@ int datagram_send_ctl(struct net *net,
 			}
 
 			*hlimit = *(int *)CMSG_DATA(cmsg);
+			if (*hlimit < -1 || *hlimit > 0xff) {
+				err = -EINVAL;
+				goto exit_f;
+			}
+
 			break;
 
 		case IPV6_TCLASS: