Patch for oops in a grabbed evdev after disconnect

[Pete Zaitcev <zaitcev@redhat.com>]

If a device was grabbed through evdev and then became disconnected,
we oops on close. This happens because input_release_device uses memory
which was freed.

Fedora enabled evdev in X11 by default recently, and now anyone who
flips a KVM oopses when they log out (Fedora bug 436659).

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>

diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
index 0727b0a..c0874a3 100644
--- a/drivers/input/evdev.c
+++ b/drivers/input/evdev.c
@@ -155,7 +155,8 @@ static int evdev_ungrab(struct evdev *evdev, struct evdev_client *client)
 
 	rcu_assign_pointer(evdev->grab, NULL);
 	synchronize_rcu();
-	input_release_device(&evdev->handle);
+	if (evdev->exist)
+		input_release_device(&evdev->handle);
 
 	return 0;
 }