Fix mmap_count.

Fix mmap_count.

Follow bug-fix by Federico Vaga in Linux "staging" version:

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);