Input: evdev - call input_flush_device() on release(), not flush()

input_flush_device() should only be called once the struct file is being
released and no open descriptors remain, but evdev_flush() was calling
it whenever a file descriptor was closed.

This caused uploaded force-feedback effects to be erased when a process
did a dup()/close() on the event FD, called system(), etc.

Call input_flush_device() from evdev_release() instead.

Reported-by: Mathieu Maret <mathieu.maret@gmail.com>
Signed-off-by: Brendan Shanks <bshanks@codeweavers.com>
Link: https://lore.kernel.org/r/20200421231003.7935-1-bshanks@codeweavers.com
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>