KVM: arm64: Get rid of reg_from/to_user()

These helpers are only used by the invariant stuff now, and while
they pretend to support non-64bit registers, this only serves as
a way to scare the casual reviewer...

Replace these helpers with our good friends get/put_user(), and
don't look back.

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