f2fs: call f2fs_balance_fs only when node was changed

If user tries to update or read data, we don't need to call f2fs_balance_fs
which triggers f2fs_gc, which increases unnecessary long latency.

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