[IPV4/IPV6] multicast: Check add_grhead() return value

add_grhead() allocates memory with GFP_ATOMIC and in at least two places skb
from it passed to skb_put() without checking.
Signed-off-by: NAlexey Dobriyan <adobriyan@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -455,6 +455,8 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ip_mc_list *pmc,
 			skb = add_grhead(skb, pmc, type, &pgr);
 			first = 0;
 		}
+		if (!skb)
+			return NULL;
 		psrc = (__be32 *)skb_put(skb, sizeof(__be32));
 		*psrc = psf->sf_inaddr;
 		scount++; stotal++;

net/ipv6/mcast.c

@@ -1582,6 +1582,8 @@ static struct sk_buff *add_grec(struct sk_buff *skb, struct ifmcaddr6 *pmc,
 			skb = add_grhead(skb, pmc, type, &pgr);
 			first = 0;
 		}
+		if (!skb)
+			return NULL;
 		psrc = (struct in6_addr *)skb_put(skb, sizeof(*psrc));
 		*psrc = psf->sf_addr;
 		scount++; stotal++;