urcu-defer: remove unnecessary memory barrier

All synchronization between queue producer/consumer is performed by the write to
"head". Before this write, none of the queued data is visible from the consumer
point of view.

Therefore, just a single wmb() is required before writing to head to ensure
correct synchronization. This matches with the rmb() after reading head on the
consumer side.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>