powerpc: Avoid comparison of unsigned long >= 0 in pfn_valid()

Rewrite comparison since all values compared are of type `unsigned long`.

Instead of using unsigned properties and rewriting the original code as:
(originally suggested by Segher Boessenkool <segher@kernel.crashing.org>)

  #define pfn_valid(pfn) \
               (((pfn) - ARCH_PFN_OFFSET) < (max_mapnr - ARCH_PFN_OFFSET))

Prefer a static inline function to make code as readable as possible.

Fix a warning (treated as error in W=1):
  arch/powerpc/include/asm/page.h:129:32: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
  #define pfn_valid(pfn)  ((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
                                  ^

Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>