Allow udp_notify() to accept read, as well as write, locks on the passed
inpcb. When directly invoking udp_notify() from udp_ctlinput(), acquire
only a read lock; we may still see write locks in udp_notify() as the
in_pcbnotifyall() routine is shared with TCP and always uses a write lock
on the inpcb being notified.

MFC after: 1 month