fix covscan issues

[dcaratti]

Covscan reported an issue in subflow_finish_connect(): checking non-nulliness of skb is reported as being useless, since we didn't do that earlier, when mptcp_get_options() was invoked.

*** CID 1463341:    (REVERSE_INULL)
/net/mptcp/subflow.c: 265 in subflow_finish_connect()
259     
260             if (subflow->mp_capable) {
261                     pr_debug("subflow=%p, remote_key=%llu", mptcp_subflow_ctx(sk),
262                              subflow->remote_key);
263                     mptcp_finish_connect(sk);
264     
>>>     CID 1463341:    (REVERSE_INULL)
>>>     Null-checking "skb" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
265                     if (skb) {
266                             pr_debug("synack seq=%u", TCP_SKB_CB(skb)->seq);
267                             subflow->ssn_offset = TCP_SKB_CB(skb)->seq;
268                     }
269             } else if (subflow->mp_join) {
270                     pr_debug("subflow=%p, thmac=%llu, remote_nonce=%u",
/net/mptcp/subflow.c: 284 in subflow_finish_connect()
278     
279                     subflow_generate_hmac(subflow->local_key, subflow->remote_key,
280                                           subflow->local_nonce,
281                                           subflow->remote_nonce,
282                                           subflow->hmac);
283     
>>>     CID 1463341:    (REVERSE_INULL)
>>>     Null-checking "skb" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
284                     if (skb)
285                             subflow->ssn_offset = TCP_SKB_CB(skb)->seq;
286     
287                     if (!mptcp_finish_join(sk))
288                             goto do_reset;
289     
}}}

[matttbe]

A fix from @dcaratti is now in our "export" branch, see 60f6bfd