reap_kill_subtree(): hold the reaper when entering it into the queue to handle later
We drop proctree_lock, which allows the process to exit while memoized
in the list to proceed.
Reported and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D35310