btrfs: switch EXTENT_BUFFER_* to enums

We can use simple enum for values that are not part of on-disk format:
extent buffer flags;
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Reviewed-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/extent_io.h

@@ -37,18 +37,22 @@
 #define EXTENT_BIO_COMPRESSED 1
 #define EXTENT_BIO_FLAG_SHIFT 16
 
-/* these are bit numbers for test/set bit */
-#define EXTENT_BUFFER_UPTODATE 0
-#define EXTENT_BUFFER_DIRTY 2
-#define EXTENT_BUFFER_CORRUPT 3
-#define EXTENT_BUFFER_READAHEAD 4	/* this got triggered by readahead */
-#define EXTENT_BUFFER_TREE_REF 5
-#define EXTENT_BUFFER_STALE 6
-#define EXTENT_BUFFER_WRITEBACK 7
-#define EXTENT_BUFFER_READ_ERR 8        /* read IO error */
-#define EXTENT_BUFFER_UNMAPPED 9
-#define EXTENT_BUFFER_IN_TREE 10
-#define EXTENT_BUFFER_WRITE_ERR 11    /* write IO error */
+enum {
+	EXTENT_BUFFER_UPTODATE,
+	EXTENT_BUFFER_DIRTY,
+	EXTENT_BUFFER_CORRUPT,
+	/* this got triggered by readahead */
+	EXTENT_BUFFER_READAHEAD,
+	EXTENT_BUFFER_TREE_REF,
+	EXTENT_BUFFER_STALE,
+	EXTENT_BUFFER_WRITEBACK,
+	/* read IO error */
+	EXTENT_BUFFER_READ_ERR,
+	EXTENT_BUFFER_UNMAPPED,
+	EXTENT_BUFFER_IN_TREE,
+	/* write IO error */
+	EXTENT_BUFFER_WRITE_ERR,
+};
 
 /* these are flags for __process_pages_contig */
 #define PAGE_UNLOCK		(1 << 0)