mptcp: Safely store sequence number when sending data

The MPTCP socket's write_seq member can be read without the msk lock
held, so use WRITE_ONCE() to store it.
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -793,7 +793,7 @@ static int mptcp_sendmsg_frag(struct sock *sk, struct sock *ssk,
 out:
 	if (!retransmission)
 		pfrag->offset += frag_truesize;
-	*write_seq += ret;
+	WRITE_ONCE(*write_seq, *write_seq + ret);
 	mptcp_subflow_ctx(ssk)->rel_write_seq += ret;
 
 	return ret;