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>
diff --git a/arch/mips/include/asm/page.h b/arch/mips/include/asm/page.h
index bf84e48..dbaec94 100644
--- a/arch/mips/include/asm/page.h
+++ b/arch/mips/include/asm/page.h
@@ -198,7 +198,10 @@
 #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)