8% performance improved by change tap interact with kernel stack

[Qin Chuanyu <qinchuanyu@xxxxxxxxxx>]

according perf test result，I found that there are 5%-8% cpu cost on 
softirq by use netif_rx_ni called in tun_get_user.

so I changed the function which cause skb transmitted more quickly.
from
	tun_get_user	->
		 netif_rx_ni(skb);
to
	tun_get_user	->
		rcu_read_lock_bh();
		netif_receive_skb(skb);
		rcu_read_unlock_bh();

The test result is as below:
	CPU: Intel(R) Xeon(R) CPU E5620  @ 2.40GHz
	NIC: intel 82599
	Host OS/Guest OS:suse11sp3
	Qemu-1.6
	netperf udp 512(VM tx)
	test model: VM->host->host

	modified before : 2.00Gbps 461146pps
	modified after  : 2.16Gbps 498782pps

8% performance gained from this change,
Is there any problem for this patch ?

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