When KN_INFLUX is set on the knote due to kqueue_register() or
kqueue_scan() unlocking the kqueue to call f_event, knote() or
knote_fork() should not skip the knote.  The knote is not going to
disappear during the influx time, and the mutual exclusion between
scan and knote() is ensured by both code pathes taking knlist lock.
The race appears since knlist lock is before kq lock, so KN_INFLUX
must be set, kq lock must be dropped and only then knlist lock can be
taken.  The window between kq unlock and knlist lock causes lost
events.

Add a flag KN_SCAN to indicate that KN_INFLUX is set in a manner safe
for the knote(), and check for it to ignore KN_INFLUX in the knote*()
as needed.  Also, in knote(), remove the lockless check for the
KN_INFLUX flag, which could also result in the lost notification.

Reported and tested by:	Kohji Okuno <okuno.kohji@jp.panasonic.com>
Discussed with:	jmg
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week