f2fs: avoid needless checkpoint in f2fs_trim_fs

The f2fs_trim_fs() doesn't need to do checkpoint if there are newly allocated
data blocks only which didn't change the critical checkpoint data such as nat
and sit entries.
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/checkpoint.c

@@ -1248,15 +1248,16 @@ int write_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	f2fs_flush_merged_bios(sbi);
 
 	/* this is the case of multiple fstrims without any changes */
-	if (cpc->reason == CP_DISCARD && !is_sbi_flag_set(sbi, SBI_IS_DIRTY)) {
-		f2fs_bug_on(sbi, NM_I(sbi)->dirty_nat_cnt);
-		f2fs_bug_on(sbi, SIT_I(sbi)->dirty_sentries);
-		f2fs_bug_on(sbi, prefree_segments(sbi));
+	if (cpc->reason == CP_DISCARD) {
+		if (NM_I(sbi)->dirty_nat_cnt == 0 &&
+				SIT_I(sbi)->dirty_sentries == 0 &&
+				prefree_segments(sbi) == 0) {
 			flush_sit_entries(sbi, cpc);
 			clear_prefree_segments(sbi, cpc);
 			unblock_operations(sbi);
 			goto out;
 		}
+	}
 
 	/*
 	 * update checkpoint pack index