This might not be the most ideal solution, since a GiB grant can be
EBUSY blocked if a single page is used by an indefinitely-blocking
UserScheme. But, the alternatives would impose lots of additional
complexity, such as increasing the PageInfo size, adding more
refcounting arrays to grants, etc.

This enables proper CoW ^ shared enforcement, so that e.g. acquiring a
new CoW page from already shared memory, or vice versa, will enforce
that they aren't both simultaneously.