f2fs: no need to check other dirty_segmap when the seg has been found

Because one dirty seg can only be mapped to one dirty_type. Otherwise, it's a bug.

Signed-off-by: Haicheng Li <haicheng.li@linux.intel.com>
[Jaegeuk Kim: modify a comment related to this patch]
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>