s390x/tcg: Pass a size to probe_write() in do_csst()

... and also call it for CONFIG_USER_ONLY. This function probably will
also need some refactoring in regards to probing, however, we'll have to
come back to that later, once cleaning up the other mem helpers.

The alignment check always makes sure that the write access falls into a
single page.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190826075112.25637-8-david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>