f2fs: remain nat cache entries for further free nid allocation

In the checkpoint flow, the f2fs investigates the total nat cache entries.
Previously, if an entry has NULL_ADDR, f2fs drops the entry and adds the
obsolete nid to the free nid list.
However, this free nid will be reused sooner, resulting in its nat entry miss.
In order to avoid this, we don't need to drop the nat cache entry at this moment.

Reviewed-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>