btrfs: async-thread: rename DFT_THRESHOLD to DEFAULT_THRESHOLD

Rename the macro so it's obvious what it means.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>