A 16-bit device number or id further distinguishes a device on the
system in addition to the device type. This is meant to be used for
the very first identification of the device for further probing. Any
further info is available by userspace mapping and probing.
Modifying task_virt_to_page() so that it takes into account page
protections. If mm0 writes to a task page that is meant to be
read-only, (e.g. the zero page) the design is broken.
Every access to a task's page will take the page fault route,
and the page fault handler will return the page instead of 0.
