mptcp: use local variable ssk in write_options

The local variable 'ssk' has been defined at the beginning of the function
mptcp_write_options(), use it instead of getting 'ssk' again.
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -1594,8 +1594,7 @@ void mptcp_write_options(struct tcphdr *th, __be32 *ptr, struct tcp_sock *tp,
 				      TCPOLEN_MPTCP_PRIO,
 				      opts->backup, TCPOPT_NOP);
 
-		MPTCP_INC_STATS(sock_net((const struct sock *)tp),
-				MPTCP_MIB_MPPRIOTX);
+		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_MPPRIOTX);
 	}
 
 mp_capable_done: