ctdb: Use provided mem_ctx for newly allocated memory

ctdb: Use provided mem_ctx for newly allocated memory

ctdb_call_local is called with a mem_ctx parameter which should be used
for newly allocated memory.

This is safe because all allocations of this context are freed before
this function returns.

Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Jeremy Allison <jra@samba.org>