kern/kern_thread.c: improve assert in thread_single_end()

The process might be stopped, which is handled by the loop that
unsuspends threads.  But the condition is not tolerated in the assert,
which insists that all threads must be unsuspended always.

Reported and tested by:	pho
Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D52293

Author: kostikbel

sys/kern/kern_thread.c

@@ -1694,8 +1694,10 @@ thread_single_end(struct proc *p, int mode)
 				thread_unlock(td);
 		}
 	}
-	KASSERT(mode != SINGLE_BOUNDARY || p->p_boundary_count == 0,
-	    ("inconsistent boundary count %d", p->p_boundary_count));
+	KASSERT(mode != SINGLE_BOUNDARY || P_SHOULDSTOP(p) ||
+	    p->p_boundary_count == 0,
+	    ("pid %d proc %p flags %#x inconsistent boundary count %d",
+	    p->p_pid, p, p->p_flag, p->p_boundary_count));
 	PROC_SUNLOCK(p);
 	wakeup(&p->p_flag);
 }