s390x: sigp: Reorder the SIGP STOP code

s390x: sigp: Reorder the SIGP STOP code

Let's wait to mark the VCPU STOPPED until the possible
STORE STATUS operation is completed, so that we know the
CPU is fully stopped and done doing anything. (When we
also clear the possible sigp_order field for STOP orders.)

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Message-Id: <20211213210919.856693-2-farman@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>