f2fs: skip searching dirty map if dirty segment is not exist

When searching victim during gc, if there are no dirty segments in
filesystem, we will still take the time to search the whole dirty segment
map, it's not needed, it's better to skip in this condition.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>