arm64: define __alloc_zeroed_user_highpage

When running the kernel with init_on_alloc=1, calling the default
implementation of __alloc_zeroed_user_highpage() from include/linux/highmem.h
leads to double-initialization of the allocated page (first by the page
allocator, then by clear_user_page().
Calling alloc_page_vma() with __GFP_ZERO, similarly to e.g. x86, seems
to be enough to ensure the user page is zeroed only once.

Signed-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>