btrfs: cleanup device states define BTRFS_DEV_STATE_FLUSH_SENT

Currently device state is being managed by each individual int
variable such as struct btrfs_device::is_tgtdev_for_dev_replace.
Instead of that declare btrfs_device::dev_state
BTRFS_DEV_STATE_FLUSH_SENT and use the bit operations.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 2ac1231..3e3ae44 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -51,6 +51,7 @@ struct btrfs_pending_bios {
 #define BTRFS_DEV_STATE_IN_FS_METADATA	(1)
 #define BTRFS_DEV_STATE_MISSING		(2)
 #define BTRFS_DEV_STATE_REPLACE_TGT	(3)
+#define BTRFS_DEV_STATE_FLUSH_SENT	(4)
 
 struct btrfs_device {
 	struct list_head dev_list;