The struct vm_layout contains fields used in __pa/__va calculations. Such
fundamental things have to be exported with EXPORT_SYMBOL to avoid
breakages of out-of-tree modules under non-GPL licenses.

Fixes: 7de0446f ("s390/boot: Make identity mapping base address explicit")
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>