datapath test: Fix wrong mempool size in ring stub

Fix wrong size being used in ring stub init function when the
alloc queue is being populated with mbufs from the mempool.

Change-Id: I3652dfafb43a9a3d0b4b47a4f00965cbc26cc620
Reviewed-by: Mark D. Gray <mark.d.gray@intel.com>
Reviewed-by: Eimear Morrissey <eimear.morrissey@intel.com>
Signed-off-by: Nicolas Pernas Maradei <nicolasx.pernas.maradei@intel.com>