netpoll: make __netpoll_cleanup non-block

Like the previous patch, slave_disable_netpoll() and __netpoll_cleanup()
may be called with read_lock() held too, so we should make them
non-block, by moving the cleanup and kfree() to call_rcu_bh() callbacks.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/8021q/vlan_dev.c b/net/8021q/vlan_dev.c
index ee4ae09..b65623f 100644
--- a/net/8021q/vlan_dev.c
+++ b/net/8021q/vlan_dev.c
@@ -704,11 +704,7 @@
 
 	info->netpoll = NULL;
 
-        /* Wait for transmitting packets to finish before freeing. */
-        synchronize_rcu_bh();
-
-        __netpoll_cleanup(netpoll);
-        kfree(netpoll);
+	__netpoll_free_rcu(netpoll);
 }
 #endif /* CONFIG_NET_POLL_CONTROLLER */