ipv6: remove leftover ip6 cookie initializer

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: 096208592b09 ("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>