ktrace: Fix a race with fork()

ktrace: Fix a race with fork()

ktrace(2) may toggle trace points in any of
1. a single process
2. all members of a process group
3. all descendents of the processes in 1 or 2

In the first two cases, we do not permit the operation if the process is
being forked or not visible. However, in case 3 we did not enforce this
restriction for descendents. As a result, the assertions about the child
in ktrprocfork() may be violated.

Move these checks into ktrops() so that they are applied consistently.

Allow KTROP_CLEAR for nascent processes. Otherwise, there is a window
where we cannot clear trace points for a nascent child if they are
inherited from the parent.

Reported by:	syzbot+d96676592978f137e05c@syzkaller.appspotmail.com
Reported by:	syzbot+7c98fcf84a4439f2817f@syzkaller.appspotmail.com
Reviewed by:	kib
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D30481