net/core: Remove unused assignment operations and variable

Although if_info_size is assigned, it has not been used. And the variable
should also be deleted.

The clang_analyzer complains as follows:

net/core/rtnetlink.c:3806: warning:

Although the value stored to 'if_info_size' is used in the enclosing
expression, the value is never actually read from 'if_info_size'.
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -3804,9 +3804,8 @@ struct sk_buff *rtmsg_ifinfo_build_skb(int type, struct net_device *dev,
 	struct net *net = dev_net(dev);
 	struct sk_buff *skb;
 	int err = -ENOBUFS;
-	size_t if_info_size;
 
-	skb = nlmsg_new((if_info_size = if_nlmsg_size(dev, 0)), flags);
+	skb = nlmsg_new(if_nlmsg_size(dev, 0), flags);
 	if (skb == NULL)
 		goto errout;