arm64: kvm: avoid CONFIG_PGTABLE_LEVELS for runtime levels

get_user_mapping_size() uses vabits_actual and CONFIG_PGTABLE_LEVELS to
provide the starting point for a table walk. This is fine for LVA, as
the number of translation levels is the same regardless of whether LVA
is enabled. However, with LPA2, this will no longer be the case, so
let's derive the number of levels from the number of VA bits directly.
Acked-by: Marc Zyngier <maz@kernel.org>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-84-ardb+git@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kvm/mmu.c

@@ -805,7 +805,7 @@ static int get_user_mapping_size(struct kvm *kvm, u64 addr)
 		.pgd		= (kvm_pteref_t)kvm->mm->pgd,
 		.ia_bits	= vabits_actual,
 		.start_level	= (KVM_PGTABLE_LAST_LEVEL -
-				   CONFIG_PGTABLE_LEVELS + 1),
+				   ARM64_HW_PGTABLE_LEVELS(pgt.ia_bits) + 1),
 		.mm_ops		= &kvm_user_mm_ops,
 	};
 	unsigned long flags;