f2fs: trigger GC when there are prefree segments

Previously, f2fs conducts SSR when free_sections() < overprovision_sections.
But, even though there are a lot of prefree segments, it can consider SSR only.
So, let's consider the number of prefree segments too for triggering SSR.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/segment.h

@@ -453,7 +453,8 @@ static inline int reserved_sections(struct f2fs_sb_info *sbi)
 
 static inline bool need_SSR(struct f2fs_sb_info *sbi)
 {
-	return (free_sections(sbi) < overprovision_sections(sbi));
+	return ((prefree_segments(sbi) / sbi->segs_per_sec)
+			+ free_sections(sbi) < overprovision_sections(sbi));
 }
 
 static inline bool has_not_enough_free_secs(struct f2fs_sb_info *sbi, int freed)