ipv6: remove leftover ip6 cookie initializer

As of the blamed commit ipc6.dontfrag is always initialized at the
start of udpv6_sendmsg, by ipcm6_init_sk, to either 0 or 1.

Later checks against -1 are no longer needed and the branches are now
dead code.

The blamed commit had removed those branches. But I had overlooked
this one case.

UDP has both a lockless fast path and a slower path for corked
requests. This branch remained in the fast path.

Fixes: 0962085 ("ipv6: replace ipcm6_init calls with ipcm6_init_sk")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250307033620.411611-2-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/ip6_output.c

@@ -2054,8 +2054,6 @@ struct sk_buff *ip6_make_skb(struct sock *sk,
 		ip6_cork_release(cork, &v6_cork);
 		return ERR_PTR(err);
 	}
-	if (ipc6->dontfrag < 0)
-		ipc6->dontfrag = inet6_test_bit(DONTFRAG, sk);
 
 	err = __ip6_append_data(sk, &queue, cork, &v6_cork,
 				&current->task_frag, getfrag, from,