KVM: arm64: vgic: Use {get,put}_user() instead of copy_{from.to}_user

Tidy-up vgic_get_common_attr() and vgic_set_common_attr() to use
{get,put}_user() instead of the more complex (and less type-safe)
copy_{from,to}_user().

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>