decnet: Fix spelling mistakes

Fix some spelling mistakes in comments:
thats  ==> that's
serivce  ==> service
varience  ==> variance
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/decnet/dn_nsp_in.c

@@ -870,7 +870,7 @@ int dn_nsp_backlog_rcv(struct sock *sk, struct sk_buff *skb)
 
 		/*
 		 * Read out ack data here, this applies equally
-		 * to data, other data, link serivce and both
+		 * to data, other data, link service and both
 		 * ack data and ack otherdata.
 		 */
 		dn_process_ack(sk, skb, other);

net/decnet/dn_nsp_out.c

@@ -179,7 +179,7 @@ static void dn_nsp_rtt(struct sock *sk, long rtt)
 		scp->nsp_srtt = 1;
 
 	/*
-	 * Add new rtt varience to smoothed varience
+	 * Add new rtt variance to smoothed varience
 	 */
 	delta >>= 1;
 	rttvar += ((((delta>0)?(delta):(-delta)) - rttvar) >> 2);

net/decnet/dn_route.c

@@ -604,7 +604,7 @@ static int dn_route_rx_short(struct sk_buff *skb)
 static int dn_route_discard(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
 	/*
-	 * I know we drop the packet here, but thats considered success in
+	 * I know we drop the packet here, but that's considered success in
 	 * this case
 	 */
 	kfree_skb(skb);