thread_unsuspend(): do not unuspend the suspended leader thread doing SINGLE_ALLPROC

markj wrote:
tdsendsignal() may unsuspend a target thread. I think there is at least
one bug there: suppose thread T is suspended in
thread_single(SINGLE_ALLPROC) when trying to kill another process with
REAP_KILL. Suppose a different thread sends SIGKILL to T->td_proc. Then,
tdsendsignal() calls thread_unsuspend(T, T->td_proc). thread_unsuspend()
incorrectly decrements T->td_proc->p_suspcount to -1.

Later, when T->td_proc exits, it will wait forever in
thread_single(SINGLE_EXIT) since T->td_proc->p_suspcount never reaches 1.

Since the thread suspension is bounded by time needed to do
thread_single(), skipping the thread_unsuspend_one() call there should
not affect signal delivery if this thread is selected as target.

Reported by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D35310