s390x: implement rrbe instruction properly

The rrbe instruction resets the reference bit in the given storage key.
So far, we merely made it a nop and also returned an invalid CC value,
so that the kernel never knew if a page actually got accessed.

This patch implements it properly, flushing the R bit and returning the
correct CC value.

Signed-off-by: Alexander Graf <agraf@suse.de>
