mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
(through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
preemption and that's sufficient on RT where interrupts are threaded.

That means we no longer need the slab_[un]lock() wrappers, so delete
them and rename the current __slab_[un]lock() to slab_[un]lock().

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>