udp: Optimize detach path

udp: Optimize detach path

If the inpcb was never put onto wildcard hash and the multicast options
were never accessed nor modified, we could directly dispose the inpcb
on detach path instead of going through all netisrs to make sure that
it is no longer being accessed.

After this commit 'kq_connect_client -u' gives ~180Kconns/s instead of
~160Kconns/s (~12% improvement).