kevent: do not check knote lists being empty before removing a knote

If a knote belongs to the list, there is no reason to check for the list
emptiness.  On the other hand, if the knote does not belong to the list,
then checking for emptiness is not enough since there might be a
different knote there.

Reviewed bu:	kevans, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D56341

sys/kern/kern_event.c

@@ -2953,8 +2953,7 @@ knote_drop_detached(struct knote *kn, struct thread *td)
 	else
 		list = &kq->kq_knhash[KN_HASH(kn->kn_id, kq->kq_knhashmask)];
 
-	if (!SLIST_EMPTY(list))
-		SLIST_REMOVE(list, kn, knote, kn_link);
+	SLIST_REMOVE(list, kn, knote, kn_link);
 	if (kn->kn_status & KN_QUEUED)
 		knote_dequeue(kn);
 	KQ_UNLOCK_FLUX(kq);

sys/kern/vfs_aio.c

@@ -2668,8 +2668,7 @@ filt_aiodetach(struct knote *kn)
 
 	knl = &kn->kn_ptr.p_aio->klist;
 	knl->kl_lock(knl->kl_lockarg);
-	if (!knlist_empty(knl))
-		knlist_remove(knl, kn, 1);
+	knlist_remove(knl, kn, 1);
 	knl->kl_unlock(knl->kl_lockarg);
 }
 
@@ -2718,8 +2717,7 @@ filt_liodetach(struct knote *kn)
 
 	knl = &kn->kn_ptr.p_lio->klist;
 	knl->kl_lock(knl->kl_lockarg);
-	if (!knlist_empty(knl))
-		knlist_remove(knl, kn, 1);
+	knlist_remove(knl, kn, 1);
 	knl->kl_unlock(knl->kl_lockarg);
 }