jbd2: fix /proc/fs/jbd2/<dev> when using an external journal

In jbd2_journal_init_dev(), we need make sure the journal structure is
fully initialzied before calling jbd2_stats_proc_init().

Reviewed-by: Andreas Dilger <andreas.dilger@oracle.com>
Signed-off-by: yangsheng <sheng.yang@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>