mptcp: Fix unsigned 'max_seq' compared with zero in mptcp_data_queue_ofo

Fixes coccicheck warnig:
net/mptcp/protocol.c:164:11-18: WARNING: Unsigned expression compared with zero: max_seq > 0

Fixes: ab174ad8 ("mptcp: move ooo skbs into msk out of order queue")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -157,11 +157,12 @@ static void mptcp_data_queue_ofo(struct mptcp_sock *msk, struct sk_buff *skb)
 	struct rb_node **p, *parent;
 	u64 seq, end_seq, max_seq;
 	struct sk_buff *skb1;
+	int space;
 
 	seq = MPTCP_SKB_CB(skb)->map_seq;
 	end_seq = MPTCP_SKB_CB(skb)->end_seq;
-	max_seq = tcp_space(sk);
-	max_seq = max_seq > 0 ? max_seq + msk->ack_seq : msk->ack_seq;
+	space = tcp_space(sk);
+	max_seq = space > 0 ? space + msk->ack_seq : msk->ack_seq;
 
 	pr_debug("msk=%p seq=%llx limit=%llx empty=%d", msk, seq, max_seq,
 		 RB_EMPTY_ROOT(&msk->out_of_order_queue));