drm/vkms: fix misuse of WARN_ON

vkms_vblank_simulate() uses WARN_ON for timing-dependent condition
(timer overrun). This is a mis-use of WARN_ON, WARN_ON must be used
to denote kernel bugs. Use pr_warn() instead.
Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: syzbot+4fc21a003c8332eb0bdd@syzkaller.appspotmail.com
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Melissa Wen <melissa.srw@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210320132840.1315853-1-dvyukov@google.com

drivers/gpu/drm/vkms/vkms_crtc.c

@@ -22,7 +22,8 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
 
 	ret_overrun = hrtimer_forward_now(&output->vblank_hrtimer,
 					  output->period_ns);
-	WARN_ON(ret_overrun != 1);
+	if (ret_overrun != 1)
+		pr_warn("%s: vblank timer overrun\n", __func__);
 
 	spin_lock(&output->lock);
 	ret = drm_crtc_handle_vblank(crtc);