mm: Implement for_each_valid_pfn() for CONFIG_FLATMEM

mm: Implement for_each_valid_pfn() for CONFIG_FLATMEM

In the FLATMEM case, the default pfn_valid() just checks that the PFN is
within the range [ ARCH_PFN_OFFSET .. ARCH_PFN_OFFSET + max_mapnr ).

The for_each_valid_pfn() function can therefore be a simple for() loop
using those as min/max respectively.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>