Input: uinput - flush all pending ff effects before destroying device

The destruction of a input device in uinput is triggered by an ioctl().
If a process tries to destroy an input device while other is uploading a
force feedback effect by evdev to the same device, they'll deadlock.
This patch fixes the problem by flushing all pending FF uploads before
destroying the device and preventing new uploads during this operation.

[dtor@mail.ru: fix logic that ensures we don't submit new requests
 to the device that is being destroyed.]
Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>