mptcp: rework mptcp_conn_request

This patch reverts the changes to the conn_request API and uses the
newly added tcp_request_sock_ops init_req method to accomplish the
same.

Note that with this approach the mptcp options are parsed twice which
adds some overhead. On the other hand this allows us to stay in sync
with upstream and once we will make it there we can easily correct
that by changing the APIs to allow passing a pointer to the mptcp
options.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Tested-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>