s390x/mmu: Move DAT protection handling out of mmu_translate_asce()

s390x/mmu: Move DAT protection handling out of mmu_translate_asce()

We'll reuse the ilen and tec definitions in mmu_translate
soon also for all other DAT exceptions we inject. Move it to the caller,
where we can later pair it up with other protection checks, like IEP.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>