kevent: Hold the knlist mutex when invoking f_event(NOTE_FORK)HEADmain

In general f_event is supposed to be called with the knlist mutex held,
so lock it earlier to follow this protocol.  Also make sure that the
update to kn_fflags is synchronized.

Lock the kqueue itself earlier in the case where the knote is activated,
to avoid locking and unlocking the kqueue twice.

PR:		291005
Reported by:	Qiu-ji Chen <chenqiuji666@gmail.com>
Reviewed by:	kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D53762

0 files changed, 0 insertions, 0 deletions