Add witness warnings to panic if a thread tries to exit while holding any

locks.

Requested by:	jeff
MFC after:	3 days

sys/kern/kern_exit.c

@@ -487,6 +487,9 @@ exit1(struct thread *td, int rv)
 	 */
 	cpu_exit(td);
 
+	WITNESS_WARN(WARN_PANIC, &proctree_lock.sx_object,
+	    "process (pid %d) exiting", p->p_pid);
+
 	PROC_LOCK(p);
 	PROC_LOCK(p->p_pptr);
 	sx_xunlock(&proctree_lock);

sys/kern/kern_kse.c

@@ -1313,6 +1313,8 @@ thread_userret(struct thread *td, struct trapframe *frame)
 		PROC_LOCK(p);
 		if (kg->kg_upsleeps)
 			wakeup(&kg->kg_completed);
+		WITNESS_WARN(WARN_PANIC, &p->p_mtx.mtx_object,
+		    "thread exiting in userret");
 		mtx_lock_spin(&sched_lock);
 		thread_stopped(p);
 		thread_exit();