f2fs: avoid frequent checkpoint during f2fs_gc

Now we're doing SSR aggressively more than ever before, so once we reach to
the reserved_segment, f2fs_balance_fs will call f2fs_gc, which triggers
checkpoint everytime. We actually must avoid that.
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -966,9 +966,11 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync, bool background)
 		 * threshold, we can make them free by checkpoint. Then, we
 		 * secure free segments which doesn't need fggc any more.
 		 */
-		ret = write_checkpoint(sbi, &cpc);
-		if (ret)
-			goto stop;
+		if (prefree_segments(sbi)) {
+			ret = write_checkpoint(sbi, &cpc);
+			if (ret)
+				goto stop;
+		}
 		if (has_not_enough_free_secs(sbi, 0, 0))
 			gc_type = FG_GC;
 	}