btrfs: switch EXTENT_BUFFER_* to enums

#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)