ipv6: use SKB_DROP_REASON_IP_OUTNOROUTES in inet6_csk_xmit()

Replace a bare kfree_skb() with a modern sk_skb_reason_drop() call,
and provide IP_OUTNOROUTES drop reason.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260511072310.1094859-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>