ext4: queue conversion after adding to inode's completed IO list

By queuing the io end on the unwritten workqueue before adding it
to our inode's list of completed IOs, I think we run the risk
of the work getting completed, and the IO freed, before we try
to add it to the inode's i_completed_io_list.

It should be safe to add it to the inode's list of completed
IOs, and -then- queue it for completion, I think.

Thanks to Dave Chinner for pointing out the race.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Jiaying Zhang <jiayingz@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>