Move LINUXKPI_PROT_VALID and LINUXKPI_CACHE_MODE_SHIFT out of the
bottom 8 bits. This allows the entire vm_prot_t to be stored in the
LinuxKPI pgprot_t.
This will be needed when we add more flags to vm_prot_t, e.g. for MTE
on arm64.
Sponsored by: Arm Ltd