Revert "Revert "locking: Remove deprecated smp_mb__() barriers""

With commit: 95dd0829f5ef (locking: Remove deprecated smp_mb__after_clear_bit())
fix in place, we no longer need to support deprecated smp_mb__() barriers.

This reverts commit a4616e0e.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>