net/mlx5: fix deadlock due to buffered slots in Rx SW ring

net/mlx5: fix deadlock due to buffered slots in Rx SW ring

When replenishing Rx ring, there're always buffered slots reserved
between consumed entries and HW owned entries. These have to be filled
with fake mbufs to protect from possible overflow rather than
optimistically expecting successful replenishment which can cause
deadlock with small-sized queue.

Fixes: fc048bd52cb7 ("net/mlx5: fix overflow of Rx SW ring")
Cc: stable@dpdk.org
Reported-by: Martin Weiser <martin.weiser@allegro-packets.com>
Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
Tested-by: Martin Weiser <martin.weiser@allegro-packets.com>