mptcp: drop unused skb in subflow_token_join_request

This patch drops the unused parameter skb in subflow_token_join_request.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>