ARM: Fix incorrect use of smp_processor_id() by syzbot report

BUG: using smp_processor_id() in preemptible [00000000] code:
syz-executor.0/15841
caller is debug_smp_processor_id+0x20/0x24
lib/smp_processor_id.c:64

The smp_processor_id() is used in a code segment when
preemption has been disabled, otherwise, when preemption
is enabled this pointer is usually no longer useful
since it may no longer point to per cpu data of the
current processor.

Reported-by: syzbot <syzbot+a7ee43e564223f195c84@syzkaller.appspotmail.com>
Fixes: f5fe12b ("ARM: spectre-v2: harden user aborts in kernel space")
Signed-off-by: Zqiang <qiang.zhang@windriver.com>

arch/arm/include/asm/system_misc.h

@@ -21,8 +21,10 @@ typedef void (*harden_branch_predictor_fn_t)(void);
 DECLARE_PER_CPU(harden_branch_predictor_fn_t, harden_branch_predictor_fn);
 static inline void harden_branch_predictor(void)
 {
+	preempt_disable();
 	harden_branch_predictor_fn_t fn = per_cpu(harden_branch_predictor_fn,
 						  smp_processor_id());
+	preempt_enable();
 	if (fn)
 		fn();
 }