net/memif: relax barrier for zero copy path

net/memif: relax barrier for zero copy path

Using 'rte_mb' to synchronize the shared ring head/tail between producer
and consumer will stall the pipeline and damage performance on the weak
memory model platforms, such like aarch64.

Relax the expensive barrier with c11 atomic with explicit memory
ordering can improve 3.6% performance on throughput.

Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Jakub Grajciar <jgrajcia@cisco.com>