ifb: use the lockless variants of skb_queue

rq and tq are both protected by tx queue lock, so we can simply use
the lockless variants of skb_queue.

skb_queue_splice_tail_init() is used instead of the open coded and slow
one.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>