KVM: arm64: nv: Respect stage-2 write permssion when setting stage-1 AF

KVM: arm64: nv: Respect stage-2 write permssion when setting stage-1 AF

Naturally, updating the Access Flag in a stage-1 descriptor requires
write permission at stage-2, although this isn't actually enforced in
KVM's software PTW.

Generate a stage-2 permission fault if the stage-1 walk attempts to
update the descriptor and its corresponding stage-2 translation lacks
write permission.

Fixes: bff8aa213dee ("KVM: arm64: Implement HW access flag management in stage-1 SW PTW")
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://msgid.link/20260108204230.677172-1-oupton@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>