staging: comedi: Use vma_pages function on vma object instead of explicit computation

This patch uses vma_pages function on vma object instead of explicit
computation.

Signed-off-by: sayli karnik <karniksayli1995@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>