Merge branch 'udp-fwd-mem-sched-on-dequeue'

Paolo Abeni says:

====================
udp: do fwd memory scheduling on dequeue

After commit 850cbadd

 ("udp: use it's own memory accounting schema"),
the udp code needs to acquire twice the receive queue spinlock on dequeue.

This patch series remove the need for the second lock at skb free time,
moving the udp memory scheduling inside the dequeue operation; the skb
destructor field is not used anymore and an additional sk argument is added
to ip_cmsg_recv_offset() to cope with null skb->sk after dequeue.

Many thanks to Eric Dumazed for suggesting pretty all much the above.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>