s390/mm,fault: replace WARN_ON_ONCE() with unreachable()

do_secure_storage_access() contains a switch statements which handles
all possible return values from get_fault_type(). Therefore remove the
pointless default case error handling and replace it with unreachable().

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: hcahca

arch/s390/mm/fault.c

@@ -618,8 +618,7 @@ void do_secure_storage_access(struct pt_regs *regs)
 			BUG();
 		break;
 	default:
-		do_fault_error(regs, VM_FAULT_BADMAP);
-		WARN_ON_ONCE(1);
+		unreachable();
 	}
 }
 NOKPROBE_SYMBOL(do_secure_storage_access);