mptcp: fix static checker warnings in mptcp_pm_add_timer

Fix the following Smatch complaint:

     net/mptcp/pm_netlink.c:213 mptcp_pm_add_timer()
     warn: variable dereferenced before check 'msk' (see line 208)

 net/mptcp/pm_netlink.c
    207          struct mptcp_sock *msk = entry->sock;
    208          struct sock *sk = (struct sock *)msk;
    209          struct net *net = sock_net(sk);
                                           ^^
 "msk" dereferenced here.

    210
    211          pr_debug("msk=%p", msk);
    212
    213          if (!msk)
                    ^^^^
 Too late.

    214                  return;
    215

Fixes: 93f323b9 ("mptcp: add a new sysctl add_addr_timeout")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Link: https://lore.kernel.org/r/078a2ef5bdc4e3b2c25ef852461692001f426495.1604976945.git.geliangtang@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/pm_netlink.c

@@ -206,7 +206,6 @@ static void mptcp_pm_add_timer(struct timer_list *timer)
 	struct mptcp_pm_add_entry *entry = from_timer(entry, timer, add_timer);
 	struct mptcp_sock *msk = entry->sock;
 	struct sock *sk = (struct sock *)msk;
-	struct net *net = sock_net(sk);
 
 	pr_debug("msk=%p", msk);
 
@@ -234,7 +233,7 @@ static void mptcp_pm_add_timer(struct timer_list *timer)
 
 	if (entry->retrans_times < ADD_ADDR_RETRANS_MAX)
 		sk_reset_timer(sk, timer,
-			       jiffies + mptcp_get_add_addr_timeout(net));
+			       jiffies + mptcp_get_add_addr_timeout(sock_net(sk)));
 
 	spin_unlock_bh(&msk->pm.lock);