If I understand your problem, you get a udp packet that contains an esp 
packet. if so, the only thing you need to do is to get rid of the udp 
"envelop", that is skip it. if so, the usual suspects are:
  - m_adj()
  - nothing: just pass to upper layer with offset set

if not so, could you explain excatly how things are? (in terms of mbuf 
parts and so on)

cheers,
mouss