s390x/tcg: fix disabling/enabling DAT

s390x/tcg: fix disabling/enabling DAT

Currently, all memory accesses go via the MMU of the address space
(primary, secondary, ...). This is bad, because we don't flush the TLB
when disabling/enabling DAT. So we could add a tlb flush. However it
is easier to simply select the MMU we already have in place for real
memory access.

All we have to do is point at the right MMU and allow to execute these
pages.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180213161240.19891-1-david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
[CH: get rid of tabs]
Signed-off-by: Cornelia Huck <cohuck@redhat.com>