f2fs: do not re-lookup nat cache with same nid

In set_node_addr, we try to lookup cached nat entry of inode and then
set flag in it.

But previously in this function, we have already grabbed nat entry with
current node id, if the node id is the same as the one of inode, we
do not need to lookup it in cache again.

So this patch adds condition judgment for reducing unneeded lookup.

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