ethdev: fix memory ordering for callback functions

ethdev: fix memory ordering for callback functions

Call back functions are registered on the control plane. They
are accessed from the data plane. Hence, correct memory orderings
should be used to avoid race conditions.

Fixes: 4dc294158cac ("ethdev: support optional Rx and Tx callbacks")
Fixes: c8231c63ddcb ("ethdev: insert Rx callback as head of list")
Cc: stable@dpdk.org
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>