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>

arch/arm64/include/asm/page.h

@@ -21,6 +21,10 @@ extern void __cpu_copy_user_page(void *to, const void *from,
 extern void copy_page(void *to, const void *from);
 extern void clear_page(void *to);
 
+#define __alloc_zeroed_user_highpage(movableflags, vma, vaddr) \
+	alloc_page_vma(GFP_HIGHUSER | __GFP_ZERO | movableflags, vma, vaddr)
+#define __HAVE_ARCH_ALLOC_ZEROED_USER_HIGHPAGE
+
 #define clear_user_page(addr,vaddr,pg)  __cpu_clear_user_page(addr, vaddr)
 #define copy_user_page(to,from,vaddr,pg) __cpu_copy_user_page(to, from, vaddr)