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()) makes it behave on PREEMPT_RT exactly as
cmpxchg_double_slab() does, so it's simpler to just redirect the usage
of the former to the latter on RT.

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>