netpoll: add RCU annotation to npinfo field

dev->npinfo is protected by RCU.

This fixes the following sparse warnings:

net/core/netpoll.c:177:48: error: incompatible types in comparison expression (different address spaces)
net/core/netpoll.c:200:35: error: incompatible types in comparison expression (different address spaces)
net/core/netpoll.c:221:35: error: incompatible types in comparison expression (different address spaces)
net/core/netpoll.c:327:18: error: incompatible types in comparison expression (different address spaces)

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1272,7 +1272,7 @@ struct net_device {
 	void (*destructor)(struct net_device *dev);
 
 #ifdef CONFIG_NETPOLL
-	struct netpoll_info	*npinfo;
+	struct netpoll_info __rcu	*npinfo;
 #endif
 
 #ifdef CONFIG_NET_NS