Introduce generic kasan_populate_zero_shadow(shadow_start,
shadow_end). This function maps kasan_zero_page to the
[shadow_start, shadow_end] addresses.

This replaces x86_64 specific populate_zero_shadow() and will
be used for ARM64 in follow on patches.

The main changes from original version are:

 * Use p?d_populate*() instead of set_p?d()
 * Use memblock allocator directly instead of vmemmap_alloc_block()
 * __pa() instead of __pa_nodebug(). __pa() causes troubles
   iff we use it before kasan_early_init(). kasan_populate_zero_shadow()
   will be used later, so we ok with __pa() here.

Signed-off-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Alexey Klimov <klimov.linux@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Keitel <dkeitel@codea...