rxrpc: do not pull udp headers on receive

Commit e6afc8ac modified the udp receive path by pulling the udp
header before queuing an skbuff onto the receive queue.

Rxrpc also calls skb_recv_datagram to dequeue an skb from a udp
socket. Modify this receive path to also no longer expect udp
headers.

Fixes: e6afc8ac

 ("udp: remove headers from UDP packets before queueing")

Signed-off-by: Willem de Bruijn <willemb@google.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>