arm/arm64: KVM: use __GFP_ZERO not memset() to get zeroed pages

Pass __GFP_ZERO to __get_free_pages() instead of calling memset()
explicitly.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>