mm/zonelist: enumerate zonelists array index

Hardcoding index to zonelists array in gfp_zonelist() is not a good
idea, let's enumerate it to improve readability.

No functional change.

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: fix CONFIG_NUMA=n build]
[n-horiguchi@ah.jp.nec.com: fix warning in comparing enumerator]
Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 1dd59ab..91f74e7 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -384,10 +384,11 @@
 
 static inline int gfp_zonelist(gfp_t flags)
 {
-	if (IS_ENABLED(CONFIG_NUMA) && unlikely(flags & __GFP_THISNODE))
-		return 1;
-
-	return 0;
+#ifdef CONFIG_NUMA
+	if (unlikely(flags & __GFP_THISNODE))
+		return ZONELIST_NOFALLBACK;
+#endif
+	return ZONELIST_FALLBACK;
 }
 
 /*