f2fs: introduce raw_nat_from_node_info() to simplfy codes

This patch introduce raw_nat_from_node_info() to simplfy some codes, and also
use exist function node_info_from_raw_nat() to do the same job.

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