tipc: remove set but not used variable 'prev'

Fixes gcc '-Wunused-but-set-variable' warning:

net/tipc/msg.c: In function 'tipc_msg_append':
net/tipc/msg.c:215:24: warning:
 variable 'prev' set but not used [-Wunused-but-set-variable]

commit 0a3e060f ("tipc: add test for Nagle algorithm effectiveness")
left behind this, remove it.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/msg.c

@@ -212,7 +212,7 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
 int tipc_msg_append(struct tipc_msg *_hdr, struct msghdr *m, int dlen,
 		    int mss, struct sk_buff_head *txq)
 {
-	struct sk_buff *skb, *prev;
+	struct sk_buff *skb;
 	int accounted, total, curr;
 	int mlen, cpy, rem = dlen;
 	struct tipc_msg *hdr;
@@ -223,7 +223,6 @@ int tipc_msg_append(struct tipc_msg *_hdr, struct msghdr *m, int dlen,
 
 	while (rem) {
 		if (!skb || skb->len >= mss) {
-			prev = skb;
 			skb = tipc_buf_acquire(mss, GFP_KERNEL);
 			if (unlikely(!skb))
 				return -ENOMEM;