net/memif: relax load of ring head for M2S zc ring
net/memif: relax load of ring head for M2S zc ring

For zero-copy M2S rings, ring->head is updated by the receiver
and eth_memif_rx_zc function is called in the context of
receiving thread. The loads in the receiver do not need to
synchronize with its own stores.

Signed-off-by: Honnappa Nagarahalli <01ccf438b971cc1d8919c8a7f18443cae9429d8b@arm.com>
Reviewed-by: Phil Yang <5a9b6465c065ff0e1fd5d2cb9eb2185e82f34247@arm.com>
Reviewed-by: Ruifeng Wang <a548d51fbc42485a8ba807711a537e88c2b59dd2@arm.com>
Reviewed-by: Jakub Grajciar <80f619e109c6efd382b3a6accd36cbd33d9669a2@cisco.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk