f2fs: fix missing cold bit during recovery

In do_recover_data, we find and update previous node pages after updating
its new block addresses.
After then, we call fill_node_footer without reset field, we erase its
cold bit so that this new cold node block is written to wrong log area.
This patch fixes not to miss its old flag.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>