This patch adds a new list to the driver's device structure. The list will
keep the file private data structures that the driver creates when a user
process opens the device.

This change is needed because it is useless to try to count how many FD
are open. Instead, track our own private data structure per open file and
once it is released, remove it from the list. As long as the list is not
empty, it means we have a user that can do something with our device.
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>