mptcp: add a new helper subflow_req_create_thmac

This patch adds a new helper named subflow_req_create_thmac, which is
extracted from subflow_token_join_request. It initializes subflow_req's
local_nonce and thmac fields, those are the more expensive to populate.

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>