ipv4: check optlen for IP_MULTICAST_IF option

Due to man page of setsockopt, if optlen is not valid, kernel should return
-EINVAL. But a simple testcase as following, errno is 0, which means setsockopt
is successful.
addr.s_addr = inet_addr("192.1.2.3");
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, 1);
printf("errno is %d\n", errno);

Xiaotian Feng([e-mail]) caught the bug. We fix it firstly checking
the availability of optlen and then dealing with the logic like other options.

Reported-by: Xiaotian Feng <[e-mail]>
Signed-off-by: Shan Wei <[e-mail]>
Acked-by: Alexey Kuznetsov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/ip_sockglue.c

···
 		 *	Check the arguments are allowable
 		 */
 
+		if (optlen < sizeof(struct in_addr))
+			goto e_inval;
+
 		err = -EFAULT;
 		if (optlen >= sizeof(struct ip_mreqn)) {
 			if (copy_from_user(&mreq, optval, sizeof(mreq)))