Enforce synchronisation across the softirqs context. On PREEMPT_RT
the softirq is preemptible. This enforces the same per-CPU BLK
semantic non-PREEMPT_RT builds have. This should not be needed
because per-CPU locks were added to avoid the per-CPU BKL.

This switch provides the old behaviour for testing reasons. Select
this if you suspect an error with preemptible softirq and want test
the old synchronized behaviour.