Staging: gasket: Use DIV_ROUND_UP

Use DIV_ROUND_UP in-kernel function to make code simple and more
understandable.

Issue found using Coccinelle.

Signed-off-by: Madhumitha Prabakaran <madhumithabiw@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>