ipv6: fix potential UAF caused by ip6_forward_proxy_check()

ip6_forward_proxy_check() calls pskb_may_pull() which might re-allocate
skb->head.

Reload ipv6_hdr() after the pskb_may_pull() call to avoid using
the freed memory.

Fixes: e21e0b5 ("[IPV6] NDISC: Handle NDP messages to proxied addresses.")
Reported-by: Damiano Melotti <melotti@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260505130056.2927197-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv6/ip6_output.c

@@ -468,6 +468,7 @@ static int ip6_forward_proxy_check(struct sk_buff *skb)
 		default:
 			break;
 		}
+		hdr = ipv6_hdr(skb);
 	}
 
 	/*
@@ -582,6 +583,8 @@ int ip6_forward(struct sk_buff *skb)
 	if (READ_ONCE(net->ipv6.devconf_all->proxy_ndp) &&
 	    pneigh_lookup(&nd_tbl, net, &hdr->daddr, skb->dev)) {
 		int proxied = ip6_forward_proxy_check(skb);
+
+		hdr = ipv6_hdr(skb);
 		if (proxied > 0) {
 			/* It's tempting to decrease the hop limit
 			 * here by 1, as we do at the end of the