Fix another race between fork(2) and PROC_REAP_KILL subtree

where we might not yet see a new child when signalling a process.
Ensure that this cannot happen by stopping all reapping subtree,
which ensures that the child is not inside a syscall, in particular
fork(2).

Reported and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D35014