[S390] __page_to_pfn warnings

For CONFIG_SPARSEMEM_VMEMMAP=y on s390 I get warnings like

init/main.c: In function 'start_kernel':
init/main.c:641: warning: format '%08lx' expects type 'long unsigned int', but argument 2 has type 'int'

The warning can be suppressed with a cast to unsigned long in the
CONFIG_SPARSEMEM_VMEMMAP=y version of __page_to_pfn.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>