net/hyperv: Use wait_event on outstanding sends during device removal

Change the busy-waiting/udelay to wait_event on outstanding sends.

Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/hyperv/hyperv_net.h b/drivers/net/hyperv/hyperv_net.h
index 4ffcd57..2857ab07 100644
--- a/drivers/net/hyperv/hyperv_net.h
+++ b/drivers/net/hyperv/hyperv_net.h
@@ -478,6 +478,7 @@
 	u32 nvsp_version;
 
 	atomic_t num_outstanding_sends;
+	wait_queue_head_t wait_drain;
 	bool start_remove;
 	bool destroy;
 	/*