f2fs: avoid needless lock for node pages when fsyncing a file

When fsync is called, sync_node_pages finds a proper direct node pages to flush.
But, it locks unrelated direct node pages together unnecessarily.

Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>