arm64: Avoid enabling KPTI unnecessarily

Commit 42c5a3b0 refactored the KPTI init code in a way that results
in the use of non-global kernel mappings even on systems that have no
need for it, and even when KPTI has been disabled explicitly via the
command line.

Ensure that this only happens when we have decided (based on the
detected system-wide CPU features) that KPTI should be enabled.

Fixes: 42c5a3b0

 ("arm64: Split kpti_install_ng_mappings()")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20231127120049.2258650-6-ardb@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>