f2fs: fix to recover cold bit of inode block during POR

Testcase to reproduce this bug:
1. mkfs.f2fs /dev/sdd
2. mount -t f2fs /dev/sdd /mnt/f2fs
3. touch /mnt/f2fs/file
4. sync
5. chattr +A /mnt/f2fs/file
6. xfs_io -f /mnt/f2fs/file -c "fsync"
7. godown /mnt/f2fs
8. umount /mnt/f2fs
9. mount -t f2fs /dev/sdd /mnt/f2fs
10. chattr -A /mnt/f2fs/file
11. xfs_io -f /mnt/f2fs/file -c "fsync"
12. umount /mnt/f2fs
13. mount -t f2fs /dev/sdd /mnt/f2fs
14. lsattr /mnt/f2fs/file

-----------------N- /mnt/f2fs/file

But actually, we expect the corrct result is:

-------A---------N- /mnt/f2fs/file

The reason is in step 9) we missed to recover cold bit flag in inode
block, so later, in fsync, we will skip write inode block due to below
condition check, result in lossing data in another SPOR.

f2fs_fsync_node_pages()
	if (!IS_DNODE(page) || !is_cold_node(page))
		continue;

Note that, I guess that some non-dir inode has already lost cold bit
during POR, so in order to reenable recovery for those inode, let's
try to recover cold bit in f2fs_iget() to save more fsynced data.

Fixes: c5667575 ("f2fs: remove unneeded set_cold_node()")
Cc: <stable@vger.kernel.org> 4.17+
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>