arm64/mm: Restructure arch_validate_flags() for extensibility

arm64/mm: Restructure arch_validate_flags() for extensibility

Currently arch_validate_flags() is written in a very non-extensible
fashion, returning immediately if MTE is not supported and writing the MTE
check as a direct return. Since we will want to add more checks for GCS
refactor the existing code to be more extensible, no functional change
intended.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-3-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>