Make knote KN_INFLUX state counted. This is final fix for the issue
closed by r310302 for knote().

If KN_INFLUX | KN_SCAN flags are set for the note passed to knote() or
knote_fork(), i.e. the knote is scanned, we might erronously clear
INFLUX when finishing notification. For normal knote() it was fixed
in r310302 simply by remembering the fact that we do not own
KN_INFLUX, since there we own knlist lock and scan thread cannot clear
KN_INFLUX until we drop the lock. For knote_fork(), the situation is
more complicated, e must drop knlist lock AKA the process lock, since
we need to register new knotes.

Change KN_INFLUX into counter and allow shared ownership of the
in-flux state between scan and knote_fork() or knote(). Both in-flux
setters need to ensure that knote is not dropped in parallel. Added
assert about kn_influx == 1 in knote_drop() verifies that in-flux state
is not shared when knote is destroyed.

Since KBI of the struct knote is changed by addition of the int
kn_influx field, reorder kn_hook and kn_hookid to fill pad on LP64
arches [1]. This keeps sizeof(struct knote) to same 128 bytes as it
was before addition of kn_influx, on amd64.

Reviewed by: markj
Suggested by: markj [1]
Tested by: pho (previous version)
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D8898