Add compiler barriers to read-side for mutex ordering

Ensure that mutex acquisition/release is not reordered wrt rcu read lock/unlock.
No impact in practice because pthreads lock/unlock primitives act as memory
barriers. But we should provide the same guarantees we are expecting from other
locking primitives, hence this modification.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>