We need to disable irqs around slab_lock() (a bit spinlock) to make it
irq-safe. The calls to slab_lock() are nested under spin_lock_irqsave() which
doesn't disable irqs on PREEMPT_RT, so add explicit disabling with PREEMPT_RT.

We also distinguish cmpxchg_double_slab() where we do the disabling explicitly
and __cmpxchg_double_slab() for contexts with already disabled irqs.  However
these context are also typically spin_lock_irqsave() thus insufficient on
PREEMPT_RT. Thus, change __cmpxchg_double_slab() to be same as
cmpxchg_double_slab() on PREEMPT_RT.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>