target/s390x: fix s390_probe_access to check PAGE_WRITE_ORG for writeability

target/s390x: fix s390_probe_access to check PAGE_WRITE_ORG for writeability

We can remove PAGE_WRITE when (internally) marking a page read-only
because it contains translated code. This can get confused when we are
executing signal return code on signal stacks.

Fixes: e56552cf07 ("target/s390x: Implement the MVPG condition-code-option bit")
Found-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-id: 20210422154427.13038-1-alex.bennee@linaro.org
Cc: Cornelia Huck <cohuck@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>