ipv6: pass proto by value to ipv6_push_nfrag_opts() and ipv6_push_frag_opts()

ipv6: pass proto by value to ipv6_push_nfrag_opts() and ipv6_push_frag_opts()

With CONFIG_STACKPROTECTOR_STRONG=y, it is better to avoid passing
a pointer to an automatic variable.

Change these exported functions to return 'u8 proto'
instead of void.

- ipv6_push_nfrag_opts()
- ipv6_push_frag_opts()

For instance, replace
	ipv6_push_frag_opts(skb, opt, &proto);
with:
	proto = ipv6_push_frag_opts(skb, opt, proto);

Note that even after this change, ip6_xmit() has to use a stack canary
because of @first_hop variable.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260130210303.3888261-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>