btrfs: convert discard stat defs to enum

Do away with the defines and use an enum as it's cleaner.
Suggested-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/free-space-cache.h

@@ -48,9 +48,11 @@ static inline bool btrfs_free_space_trimming_bitmap(
  * to make it clear what we're doing.  An example is discard_extents in
  * btrfs_free_space_ctl.
  */
-#define BTRFS_STAT_NR_ENTRIES	2
-#define BTRFS_STAT_CURR		0
-#define BTRFS_STAT_PREV		1
+enum {
+	BTRFS_STAT_CURR,
+	BTRFS_STAT_PREV,
+	BTRFS_STAT_NR_ENTRIES,
+};
 
 struct btrfs_free_space_ctl {
 	spinlock_t tree_lock;