From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
There is stale definition of pfn_valid() for DISCONTINGMEM memory model
guarded !FLATMEM && !SPARSEMEM && NUMA ifdefery.
Remove everything but definition of pfn_valid() for FLATMEM.
Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>