MIPS: Fix modpost error in modules attepting to use virt_addr_valid().

ERROR: "min_low_pfn" [drivers/block/aoe/aoe.ko] undefined!

Fixed by moving the implementation of virt_addr_valid() into the kernel
proper and exporting it which removes the pains of an inline or macro
implementation.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/page.h

@@ -198,7 +198,10 @@ typedef struct { unsigned long pgprot; } pgprot_t;
 #endif
 
 #define virt_to_page(kaddr)	pfn_to_page(PFN_DOWN(virt_to_phys(kaddr)))
-#define virt_addr_valid(kaddr)	pfn_valid(PFN_DOWN(virt_to_phys(kaddr)))
+
+extern int __virt_addr_valid(const volatile void *kaddr);
+#define virt_addr_valid(kaddr)						\
+	__virt_addr_valid((const volatile void *) (kaddr))
 
 #define VM_DATA_DEFAULT_FLAGS	(VM_READ | VM_WRITE | VM_EXEC | \
 				 VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)

arch/mips/mm/ioremap.c

@@ -190,3 +190,9 @@ void __iounmap(const volatile void __iomem *addr)
 
 EXPORT_SYMBOL(__ioremap);
 EXPORT_SYMBOL(__iounmap);
+
+int __virt_addr_valid(const volatile void *kaddr)
+{
+	return pfn_valid(PFN_DOWN(virt_to_phys(kaddr)));
+}
+EXPORT_SYMBOL_GPL(__virt_addr_valid);