Staging: comedi: For COMEDI_BUFINFO, check access to command

Don't allow COMEDI_BUFINFO ioctl if some other file object has locked
the subdevice or has an active command.  If there is no active command,
just report back the last buffer position.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>