libnet dssync: fix memory allocation for error/result messages.

libnet dssync: fix memory allocation for error/result messages.

Use the libnet_dssync_context as a talloc context for the
result_message and error_message string members.
Using the passed in mem_ctx makes the implicit assumption
that mem_ctx is at least as long-lived as the libnet_dssync_context,
which is wrong.

Michael
(This used to be commit 635baf6b7d2a1822ceb48aa4bc47569ef19d51cc)