dapltest: Implement a malloc() threshold for the completion reaping.

dapltest: Implement a malloc() threshold for the completion reaping.

change byte vector allocation to stack in functions:
  DT_handle_send_op, DT_handle_rdma_op & DT_handle_recv_op.

When allocation size is under the threshold, use a stack local
allocation instead of malloc/free.  Move redundant bzero() to
be called only in the case of using local stack allocation as
DT_Mdep_malloc() already does a bzero(). Consolidate error handling
return and free()check to a single point by using goto.

Signed-off-by: Stan Smith <stan.smith@intel.com>