mm/swap_slots.c: don't disable preemption while taking the per-CPU cache

mm/swap_slots.c: don't disable preemption while taking the per-CPU cache

get_cpu_var() disables preemption and returns the per-CPU version of the
variable.  Disabling preemption is useful to ensure atomic access to the
variable within the critical section.

In this case however, after the per-CPU version of the variable is
obtained the ->free_lock is acquired.  For that reason it seems the raw
accessor could be used.  It only seems that ->slots_ret should be
retested (because with disabled preemption this variable can not be set
to NULL otherwise).

This popped up during PREEMPT-RT testing because it tries to take
spinlocks in a preempt disabled section.  In RT, spinlocks can sleep.

Link: http://lkml.kernel.org/r/20170623114755.2ebxdysacvgxzott@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ying Huang <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>