f2fs: avoid redundant clean nat entry move in lru list

__lookup_nat_cache follows LRU manner to move clean nat entry, when nat
entries are going to be dirty, no need to move them to tail of lru list.
Introduce a parameter 'for_dirty' to avoid it.

Signed-off-by: wangzijie <wangzijie1@honor.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>