btrfs: async-thread: convert defines to enums

Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/async-thread.c

@@ -12,9 +12,11 @@
 #include "async-thread.h"
 #include "ctree.h"
 
-#define WORK_DONE_BIT 0
-#define WORK_ORDER_DONE_BIT 1
-#define WORK_HIGH_PRIO_BIT 2
+enum {
+	WORK_DONE_BIT,
+	WORK_ORDER_DONE_BIT,
+	WORK_HIGH_PRIO_BIT,
+};
 
 #define NO_THRESHOLD (-1)
 #define DFT_THRESHOLD (32)