arm64/mm: Avoid ID mapping of kpti flag if it is no longer needed

arm64_use_ng_mappings will be set to 'true' by the early boot code if it
decides to use non-global (nG) attributes for all kernel mappings,
typically when enabling KASLR on a system that does not implement E0PD.

In this case, the G-to-nG update routines are never called, and so there
is no reason to create the writable mapping of the associated status
flag in the ID map.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240301104046.1234309-6-ardb+git@google.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>