asm-generic: add an optional pfn_valid check to page_to_phys

page_to_pfn is usually implemented by pointer arithmetics on the memory
map, which means that bogus input can lead to even more bogus output.

Powerpc had a pfn_valid check on the intermediate pfn in the page_to_phys
implementation when CONFIG_DEBUG_VIRTUAL is defined, which seems
generally useful, so add that to the generic version.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>