net/memif: fix extra mbuf refcnt update in zero copy Tx

[ upstream commit aa3e97fcb55d7b68fef864aa76078bdae375ad3d ]

The refcnt update of stored mbufs in memif driver is redundant since
those mbufs are only freed in eth_memif_tx_zc(). No other place can
free those stored mbufs quietly. By removing this redundant update
single core dpdk memif performance can be improved by 7.5%.

testpmd stats on Arm Neoverse N1 (Ampere Altra)
+-----------------------------+-----------------------+
|        | With refcnt update | Without refcnt update |
+--------+--------------------+-----------------------+
| Rx-pps |      2748851       |         2955487       |
+--------+--------------------+-----------------------+
| Tx-pps |      2748812       |         2955436       |
+--------+--------------------+-----------------------+

Fixes: 43b815d ("net/memif: support zero-copy slave")

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Signed-off-by: Wathsala Vithanage <wathsala.vithanage@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Ferruh Yigit <ferruh.yigit@amd.com>

.mailmap

@@ -1451,6 +1451,7 @@ Walter Heymans <walter.heymans@corigine.com>
 Wang Sheng-Hui <shhuiw@gmail.com>
 Wangyu (Eric) <seven.wangyu@huawei.com>
 Waterman Cao <waterman.cao@intel.com>
+Wathsala Vithanage <wathsala.vithanage@arm.com>
 Weichun Chen <weichunx.chen@intel.com>
 Wei Dai <wei.dai@intel.com>
 Weifeng Li <liweifeng96@126.com>

drivers/net/memif/rte_eth_memif.c

@@ -261,8 +261,6 @@ memif_free_stored_mbufs(struct pmd_process_private *proc_private, struct memif_q
 	cur_tail = __atomic_load_n(&ring->tail, __ATOMIC_ACQUIRE);
 	while (mq->last_tail != cur_tail) {
 		RTE_MBUF_PREFETCH_TO_FREE(mq->buffers[(mq->last_tail + 1) & mask]);
-		/* Decrement refcnt and free mbuf. (current segment) */
-		rte_mbuf_refcnt_update(mq->buffers[mq->last_tail & mask], -1);
 		rte_pktmbuf_free_seg(mq->buffers[mq->last_tail & mask]);
 		mq->last_tail++;
 	}
@@ -707,10 +705,6 @@ memif_tx_one_zc(struct pmd_process_private *proc_private, struct memif_queue *mq
 next_in_chain:
 	/* store pointer to mbuf to free it later */
 	mq->buffers[slot & mask] = mbuf;
-	/* Increment refcnt to make sure the buffer is not freed before server
-	 * receives it. (current segment)
-	 */
-	rte_mbuf_refcnt_update(mbuf, 1);
 	/* populate descriptor */
 	d0 = &ring->desc[slot & mask];
 	d0->length = rte_pktmbuf_data_len(mbuf);