We're going to use this flag EXTENT_DEFRAG to indicate which range
belongs to defragment so that we can implement snapshow-aware defrag:

We set the EXTENT_DEFRAG flag when dirtying the extents that need
defragmented, so later on writeback thread can differentiate between
normal writeback and writeback started by defragmentation.
Original-Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>