arm64: kasan: ensure that the KASAN zero page is mapped read-only

arm64: kasan: ensure that the KASAN zero page is mapped read-only

When switching from the early KASAN shadow region, which maps the
entire shadow space read-write, to the permanent KASAN shadow region,
which uses a zero page to shadow regions that are not subject to
instrumentation, the lowest level table kasan_zero_pte[] may be
reused unmodified, which means that the mappings of the zero page
that it contains will still be read-write.

So update it explicitly to map the zero page read only when we
activate the permanent mapping.

Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
(cherry picked from commit 7b1af9795773d745c2a8c7d4ca5f2936e8b6adfb)
Signed-off-by: Alex Shi <alex.shi@linaro.org>