kern_exit.c: do not leak reaper bitmap busy bits

When re-assigning p_reapsubtree value, we must check if the result of
obliterating the old value would free the corresponding pid in the
proc_id_reap bitmap, and indeed free it when needed.

PR: 289917
Reported by: avg
Reviewed by: avg, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D52814