KVM: arm64: Use get_page() instead of kvm_get_pfn()

When mapping a THP, we are guaranteed that the page isn't reserved,
and we can safely avoid the kvm_is_reserved_pfn() call.

Replace kvm_get_pfn() with get_page(pfn_to_page()).

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210726153552.1535838-6-maz@kernel.org