f2fs: introduce help function {create,destroy}_flush_cmd_control

Introduce help function {create,destroy}_flush_cmd_control to clean up
the create/destory flush merge operation.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>