mm: Define VM_HIGH_ARCH_6

The addition of protection keys means that on arm64 we now use all of the
currently defined VM_HIGH_ARCH_x bits. In order to allow us to allocate a
new flag for GCS pages define VM_HIGH_ARCH_6.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-2-222b78d87eee@kernel.org


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