[S390] BUG to BUG_ON changes

Signed-off-by: Stoyan Gaydarov <stoyboyker@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
index 46fc981..580abb5 100644
--- a/arch/s390/kernel/setup.c
+++ b/arch/s390/kernel/setup.c
@@ -134,8 +134,7 @@
 
 	atomic_inc(&init_mm.mm_count);
 	current->active_mm = &init_mm;
-        if (current->mm)
-                BUG();
+	BUG_ON(current->mm);
         enter_lazy_tlb(&init_mm, current);
 }