vfs: Add event notification points

The INOTIFY macro and its variants check if the vnode is being watched,
and if so, calls into a slow path which adds an event to one or more
inotify descriptors. Most of these events correspond to EVFILT_VNODE
events as well, and are added to VOP *_post hooks.

Reviewed by: kib
MFC after: 3 months
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50315