net/ipv4: Revert use of struct_size() helper

Revert the use of structr_size() and stay with IP_MSFILTER_SIZE() for
now, as in this case, the size of struct ip_msfilter didn't change with
the addition of the flexible array imsf_slist_flex[]. So, if we use
struct_size() we will be allocating and calculating the size of
struct ip_msfilter with one too many items for imsf_slist_flex[].

We might use struct_size() in the future, but for now let's stay
with IP_MSFILTER_SIZE().

Fixes: 2d3e5caf96b9 ("net/ipv4: Replace one-element array with flexible-array member")
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/igmp.c

···
 	copycount = count < msf->imsf_numsrc ? count : msf->imsf_numsrc;
 	len = flex_array_size(psl, sl_addr, copycount);
 	msf->imsf_numsrc = count;
-	if (put_user(struct_size(optval, imsf_slist_flex, copycount), optlen) ||
-	    copy_to_user(optval, msf, struct_size(optval, imsf_slist_flex, 0))) {
+	if (put_user(IP_MSFILTER_SIZE(copycount), optlen) ||
+	    copy_to_user(optval, msf, IP_MSFILTER_SIZE(0))) {
 		return -EFAULT;
 	}
 	if (len &&

net/ipv4/ip_sockglue.c

···
 	struct sockaddr_in *psin;
 	int err, i;
 
-	msf = kmalloc(struct_size(msf, imsf_slist_flex, numsrc), GFP_KERNEL);
+	msf = kmalloc(IP_MSFILTER_SIZE(numsrc), GFP_KERNEL);
 	if (!msf)
 		return -ENOBUFS;
 
···
 	{
 		struct ip_msfilter *msf;
 
-		if (optlen < struct_size(msf, imsf_slist_flex, 0))
+		if (optlen < IP_MSFILTER_SIZE(0))
 			goto e_inval;
 		if (optlen > sysctl_optmem_max) {
 			err = -ENOBUFS;
···
 			err = -ENOBUFS;
 			break;
 		}
-		if (struct_size(msf, imsf_slist_flex, msf->imsf_numsrc) >
-		    optlen) {
+		if (IP_MSFILTER_SIZE(msf->imsf_numsrc) > optlen) {
 			kfree(msf);
 			err = -EINVAL;
 			break;
···
 	{
 		struct ip_msfilter msf;
 
-		if (len < struct_size(&msf, imsf_slist_flex, 0)) {
+		if (len < IP_MSFILTER_SIZE(0)) {
 			err = -EINVAL;
 			goto out;
 		}
-		if (copy_from_user(&msf, optval,
-				   struct_size(&msf, imsf_slist_flex, 0))) {
+		if (copy_from_user(&msf, optval, IP_MSFILTER_SIZE(0))) {
 			err = -EFAULT;
 			goto out;
 		}