netpoll: Add netpoll_tx_running

This patch adds the helper netpoll_tx_running for use within
ndo_start_xmit.  It returns non-zero if ndo_start_xmit is being
invoked by netpoll, and zero otherwise.

This is currently implemented by simply looking at the hardirq
count.  This is because for all non-netpoll uses of ndo_start_xmit,
IRQs must be enabled while netpoll always disables IRQs before
calling ndo_start_xmit.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netpoll.h

@@ -116,6 +116,11 @@ static inline void netpoll_poll_unlock(void *have)
 	}
 }
 
+static inline int netpoll_tx_running(struct net_device *dev)
+{
+	return irqs_disabled();
+}
+
 #else
 static inline int netpoll_rx(struct sk_buff *skb)
 {
@@ -139,6 +144,10 @@ static inline void netpoll_poll_unlock(void *have)
 static inline void netpoll_netdev_init(struct net_device *dev)
 {
 }
+static inline int netpoll_tx_running(struct net_device *dev)
+{
+	return 0;
+}
 #endif
 
 #endif