KVM: arm64: Fix benign bug with incorrect use of VA_BITS

get_user_mapping_size() uses kvm's pgtable library to walk a user space
page table created by the kernel, and in doing so, passes metadata
that the library needs, including ia_bits, which defines the size of the
input address.

For the case where the kernel is compiled for 52 VA bits but runs on HW
that does not support LVA, it will fall back to 48 VA bits at runtime.
Therefore we must use vabits_actual rather than VA_BITS to get the true
address size.

This is benign in the current code base because the pgtable library only
uses it for error checking.

Fixes: 6011cf68 ("KVM: arm64: Walk userspace page tables to compute the THP mapping size")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221205114031.3972780-1-ryan.roberts@arm.com