s390: TASK_SIZE for kernel threads

commit fb94a687 upstream.

Return a sensible value if TASK_SIZE if called from a kernel thread.

This gets us around an issue with copy_mount_options that does a magic
size calculation "TASK_SIZE - (unsigned long)data" while in a kernel
thread and data pointing to kernel space.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>

arch/s390/include/asm/processor.h

@@ -48,7 +48,8 @@ extern void execve_tail(void);
 
 #else /* CONFIG_64BIT */
 
-#define TASK_SIZE_OF(tsk)	((tsk)->mm->context.asce_limit)
+#define TASK_SIZE_OF(tsk)	((tsk)->mm ? \
+				 (tsk)->mm->context.asce_limit : TASK_MAX_SIZE)
 #define TASK_UNMAPPED_BASE	(test_thread_flag(TIF_31BIT) ? \
 					(1UL << 30) : (1UL << 41))
 #define TASK_SIZE		TASK_SIZE_OF(current)