[S390] init task memory faults.

Lock for mmap_sem is missing on page fault retry for init task
when it fails due to out of memory.
Signed-off-by: Akinobu Mita <mita@miraclelinux.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/fault.c

@@ -355,6 +355,7 @@ do_exception(struct pt_regs *regs, unsigned long error_code, int is_protection)
 	up_read(&mm->mmap_sem);
 	if (tsk->pid == 1) {
 		yield();
+		down_read(&mm->mmap_sem);
 		goto survive;
 	}
 	printk("VM: killing process %s\n", tsk->comm);