MIPS: Rewrite pfn_valid to work in modules, too.

This fixes:

  MODPOST 393 modules
ERROR: "min_low_pfn" [arch/mips/kvm/kvm.ko] undefined!
make[3]: *** [__modpost] Error 1

It would have been possible to just export min_low_pfn but in the end
pfn_valid should return 1 for any pfn argument for which a struct page
exists so using min_low_pfn was wrong anyway.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/page.h

@@ -171,14 +171,13 @@ typedef struct { unsigned long pgprot; } pgprot_t;
 
 #ifdef CONFIG_FLATMEM
 
-#define pfn_valid(pfn)							\
-({									\
-	unsigned long __pfn = (pfn);					\
-	/* avoid <linux/bootmem.h> include hell */			\
-	extern unsigned long min_low_pfn;				\
-									\
-	__pfn >= min_low_pfn && __pfn < max_mapnr;			\
-})
+static inline int pfn_valid(unsigned long pfn)
+{
+	/* avoid <linux/mm.h> include hell */
+	extern unsigned long max_mapnr;
+
+	return pfn >= ARCH_PFN_OFFSET && pfn < max_mapnr;
+}
 
 #elif defined(CONFIG_SPARSEMEM)