net/mlx5: fix overflow of Rx SW ring

net/mlx5: fix overflow of Rx SW ring

If vectorized Rx burst is short of mbufs in replenishment, Rx SW ring can
overflow as the Rx burst handles 4 packets in a loop. This is because the
function fills SW ring and its mbufs first and checks validity of
each completion later. So, there should be some buffer slots at the tail of
the ring to protect mbufs which are already owned by application.

Fixes: 6cb559d67b83 ("net/mlx5: add vectorized Rx/Tx burst for x86")
Cc: stable@dpdk.org
Reported-by: Martin Weiser <martin.weiser@allegro-packets.com>
Signed-off-by: Yongseok Koh <yskoh@mellanox.com>