md/raid5: play nice with PREEMPT_RT

raid_run_ops() relies on the implicitly disabled preemption for
its percpu ops, although this is really about CPU locality. This
breaks RT semantics as it can take regular (and thus sleeping)
spinlocks, such as stripe_lock.

Add a local_lock such that non-RT does not change and continues
to be just map to preempt_disable/enable, but makes RT happy as
the region will use a per-CPU spinlock and thus be preemptible
and still guarantee CPU locality.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Song Liu <songliubraving@fb.com>