net-next: gro: Fix use of skb_gro_header_slow

In the cited commit, the function ipv6_gro_receive was accidentally
changed to use skb_gro_header_slow, without attempting the fast path.
Fix it.

Fixes: 35ffb665 ("net: gro: skb_gro_header helper function")
Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
Link: https://lore.kernel.org/r/20220911184835.GA105063@debianSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv6/ip6_offload.c

@@ -219,7 +219,7 @@ INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
 
 	off = skb_gro_offset(skb);
 	hlen = off + sizeof(*iph);
-	iph = skb_gro_header_slow(skb, hlen, off);
+	iph = skb_gro_header(skb, hlen, off);
 	if (unlikely(!iph))
 		goto out;