staging: gasket: gasket_mmap use PAGE_MASK

gasket_mmap use PAGE_MASK, instead of performing math on PAGE_SIZE, for
simplicity and clarity.

Reported-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Simon Que <sque@chromium.org>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>