Hold a write lock across udp_notify()

With the new route cache feature udp_notify() will modify the inp when it
needs to invalidate the route cache. Ensure that we hold a write lock on
the inp before calling the function to ensure that multiple threads don't
race while trying to invalidate the cache (which previously lead to a page
fault).

Differential Revision: https://reviews.freebsd.org/D17246
Reviewed by: sbruno, bz, karels
Sponsored by: Dell EMC Isilon
Approved by: re (gjb)