Introduction of write-combined memory type mappings.

Introduction of VR_WRITE_COMBINED, PTATTR_WRITE_COMBINED and modification
to the memobj's get_page() interface so that Linux communicates back mapping
flags (such as write-combined).

arch/x86/kernel/include/arch-memory.h

@@ -147,6 +147,7 @@ enum ihk_mc_pt_attribute {
 	PTATTR_NO_EXECUTE = 0x8000000000000000,
 	PTATTR_UNCACHABLE = 0x10000,
 	PTATTR_FOR_USER   = 0x20000,
+	PTATTR_WRITE_COMBINED = 0x40000,
 };
 
 static inline int pte_is_null(pte_t *ptep)