s390/kasan: support preemptible kernel build

When the kernel is built with:
CONFIG_PREEMPT=y
CONFIG_PREEMPT_COUNT=y
"stfle" function used by kasan initialization code makes additional
call to preempt_count_add/preempt_count_sub. To avoid removing kasan
instrumentation from sched code where those functions leave split stfle
function and provide __stfle variant without preemption handling to be
used by Kasan.

Reported-by: Benjamin Block <bblock@linux.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/kasan_init.c

@@ -222,8 +222,8 @@ static void __init kasan_enable_dat(void)
 
 static void __init kasan_early_detect_facilities(void)
 {
-	stfle(S390_lowcore.stfle_fac_list,
-	      ARRAY_SIZE(S390_lowcore.stfle_fac_list));
+	__stfle(S390_lowcore.stfle_fac_list,
+		ARRAY_SIZE(S390_lowcore.stfle_fac_list));
 	if (test_facility(8)) {
 		has_edat = true;
 		__ctl_set_bit(0, 23);