ipv6: fix possible UAF in ip6_finish_output2()

ipv6: fix possible UAF in ip6_finish_output2()

If skb_expand_head() returns NULL, skb has been freed
and associated dst/idev could also have been freed.

We need to hold rcu_read_lock() to make sure the dst and
associated idev are alive.

Fixes: 5796015fa968 ("ipv6: allocate enough headroom in ip6_finish_output2()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Vasily Averin <vasily.averin@linux.dev>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240820160859.3786976-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>