net: fix two sparse warnings introduced by IGMP/MLD parsing exports

> net/core/skbuff.c:4108:13: sparse: incorrect type in assignment (different base types)
> net/ipv6/mcast_snoop.c:63 ipv6_mc_check_exthdrs() warn: unsigned 'offset' is never less than zero.

Introduced by 9afd85c9e4552b276e2f4cfefd622bdeeffbbf26
("net: Export IGMP/MLD message validation code")

Reported-by: kbuild test robot <[e-mail]>
Signed-off-by: Linus Lüssing <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/skbuff.c

···
 {
 	struct sk_buff *skb_chk;
 	unsigned int offset = skb_transport_offset(skb);
-	int ret;
+	__sum16 ret;
 
 	skb_chk = skb_checksum_maybe_trim(skb, transport_len);
 	if (!skb_chk)

net/ipv6/mcast_snoop.c

···
 static int ipv6_mc_check_exthdrs(struct sk_buff *skb)
 {
 	const struct ipv6hdr *ip6h;
-	unsigned int offset;
+	int offset;
 	u8 nexthdr;
 	__be16 frag_off;