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

arch/arm64/mm/mmu.c

@@ -777,7 +777,7 @@ static void __init create_idmap(void)
 		       IDMAP_ROOT_LEVEL, (pte_t *)idmap_pg_dir, false,
 		       __phys_to_virt(ptep) - ptep);
 
-	if (IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0)) {
+	if (IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0) && !arm64_use_ng_mappings) {
 		extern u32 __idmap_kpti_flag;
 		u64 pa = __pa_symbol(&__idmap_kpti_flag);