arm64: mm: apply __ro_after_init to memory_limit

This variable is only set during initialization, so mark with
__ro_after_init.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20211215064559.2843555-1-peng.fan@oss.nxp.com


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