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 9afd85c9
("net: Export IGMP/MLD message validation code")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -4093,7 +4093,7 @@ struct sk_buff *skb_checksum_trimmed(struct sk_buff *skb,
 {
 	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

@@ -47,7 +47,7 @@ static int ipv6_mc_check_ip6hdr(struct sk_buff *skb)
 static int ipv6_mc_check_exthdrs(struct sk_buff *skb)
 {
 	const struct ipv6hdr *ip6h;
-	unsigned int offset;
+	int offset;
 	u8 nexthdr;
 	__be16 frag_off;