Staging: comedi: fix mmap_count

In comedi_fops, mmap_count is decremented at comedi_vm_ops->close but
it is not incremented at comedi_vm_ops->open. This may result in a negative
counter.  The patch introduces the open method to keep the counter
consistent.

The bug was triggerd by this sample code:

        mmap(0, ...., comedi_fd);
        fork();
        exit(0);

Acked-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>