staging: comedi: support vm_access_process for mmap'd buffer

If a process that has mmap'd a COMEDI buffer is being run under a
debugger such as GDB, the buffer contents are inaccessible from the
debugger.  Support the `access()` VM operation to allow the buffer
contents to be accessed by another process.

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