f2fs: avoid unnecessary checkpoints

During the f2fs_put_super procedure, we don't need to conduct checkpoint all
the time, since we don't need to do that if superblock is clean.
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/super.c

@@ -372,6 +372,8 @@ static void f2fs_put_super(struct super_block *sb)
 	f2fs_destroy_stats(sbi);
 	stop_gc_thread(sbi);
 
+	/* We don't need to do checkpoint when it's clean */
+	if (sbi->s_dirty && get_pages(sbi, F2FS_DIRTY_NODES))
 		write_checkpoint(sbi, true);
 
 	iput(sbi->node_inode);