KVM: arm64: vgic-v3: Fix vcpu index comparison

When handling an error at the point where we try and register
all the redistributors, we unregister all the previously
registered frames by counting down from the failing index.

However, the way the code is written relies on that index
being a signed value. Which won't be true once we switch to
an xarray-based vcpu set.

Since this code is pretty awkward the first place, and that the
failure mode is hard to spot, rewrite this loop to iterate
over the vcpus upwards rather than downwards.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211216104526.1482124-1-maz@kernel.org