When testing IPv6 fragmentation uisng net/mlx5 driver we noticed the indirect
buffer is not returning to the pool and will eventually cause indirect pool to
be exausted.

when using intel nic with iavf driver with same application code the issue is
not seen. There is also no issue for pcap driver.

when review code
https://github.com/DPDK/dpdk/blob/main/drivers/net/mlx5/mlx5_tx.h function
mlx5_tx_free_mbuf(). Seems in rte_pktmbuf_prefree_seg(*pkts) the pkts->next
will be set to NULL for indirect buffer which is linked to the direct buffer.

Test is done by explicitly putting the indirect buffer back to mem pool at end
of function mlx5_tx_free_mbuf().