net/mlx5: prepare vector Rx ring at setup time

net/mlx5: prepare vector Rx ring at setup time

To use the vector, it needs to add to the PMD Rx mbuf ring four extra mbuf
to avoid memory corruption.  This additional mbuf are added on dev_start()
whereas all other mbuf are allocated on queue setup.

This patch brings this allocation back to the same place as other mbuf
allocation.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>