s390: TASK_SIZE for kernel threads

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.

Cc: <stable@vger.kernel.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>