rculfhash: Ensure future-proof memory barrier semantic consistency

Use cmm_smp_mb__before_uatomic_or() prior to the uatomic_or() in
_rcu_lfht_del() to ensure correct memory barrier semantic when we relax
(in the future) the barrier implementation of some architectures.

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