nios2: Convert pfn_valid to static inline

When compiling with 'W=1' warnings such as the following occur all over
the place:

  ./include/linux/pfn.h:21:22: warning: comparison of unsigned expression
    >= 0 is always true [-Wtype-limits]

which is due to ARCH_PFN_OFFSET being 0 by default
(if CONFIG_NIOS2_MEM_BASE is not changed in the Kconfig). Fix these
warnings by making pfn_valid a static inline function.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>

arch/nios2/include/asm/page.h

@@ -85,8 +85,15 @@ extern struct page *mem_map;
 	((void *)(((page) - mem_map) << PAGE_SHIFT) + PAGE_OFFSET)
 
 # define pfn_to_kaddr(pfn)	__va((pfn) << PAGE_SHIFT)
-# define pfn_valid(pfn)		((pfn) >= ARCH_PFN_OFFSET &&	\
-					(pfn) < max_mapnr)
+
+static inline bool pfn_valid(unsigned long pfn)
+{
+	/* avoid <linux/mm.h> include hell */
+	extern unsigned long max_mapnr;
+	unsigned long pfn_offset = ARCH_PFN_OFFSET;
+
+	return pfn >= pfn_offset && pfn < max_mapnr;
+}
 
 # define virt_to_page(vaddr)	pfn_to_page(PFN_DOWN(virt_to_phys(vaddr)))
 # define virt_addr_valid(vaddr)	pfn_valid(PFN_DOWN(virt_to_phys(vaddr)))