ext4: recover csum seed of tmp_inode after migrating to extents

ext4: recover csum seed of tmp_inode after migrating to extents

When migrating to extents, the checksum seed of temporary inode
need to be replaced by inode's, otherwise the inode checksums
will be incorrect when swapping the inodes data.

However, the temporary inode can not match it's checksum to
itself since it has lost it's own checksum seed.

mkfs.ext4 -F /dev/sdc
mount /dev/sdc /mnt/sdc
xfs_io -fc "pwrite 4k 4k" -c "fsync" /mnt/sdc/testfile
chattr -e /mnt/sdc/testfile
chattr +e /mnt/sdc/testfile
umount /dev/sdc
fsck -fn /dev/sdc

========
...
Pass 1: Checking inodes, blocks, and sizes
Inode 13 passes checks, but checksum does not match inode.  Fix? no
...
========

The fix is simple, save the checksum seed of temporary inode, and
recover it after migrating to extents.

Fixes: e81c9302a6c3 ("ext4: set csum seed in tmp inode while migrating to extents")
Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220617062515.2113438-1-lilingfeng3@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>