arm64: mm: avoid virt_to_page() translation for the zero page

The zero page is statically allocated, so grab its struct page pointer
without using virt_to_page(), which will be restricted to the linear
mapping later.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>