mm/slub: simplify __cmpxchg_double_slab()

The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
make it behave exactly as cmpxchg_double_slab(), so it's simpler to just
make sure the latter is called on RT.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>