tune2fs: update journal super block when changing UUID for fs.

tune2fs: update journal super block when changing UUID for fs.

Using -U option you can change the UUID for fs, however it will not work
for journal device, since it have a copy of this UUID inside jsb (i.e.
journal super block). So copy UUID on change into that block.

Here is the initial thread:
http://comments.gmane.org/gmane.comp.file-systems.ext4/44532

You can reproduce this by executing following commands:
$ fallocate -l100M /tmp/dev
$ fallocate -l100M /tmp/journal
$ sudo /sbin/losetup /dev/loop1 /tmp/dev
$ sudo /sbin/losetup /dev/loop0 /tmp/journal
$ mke2fs -O journal_dev /tmp/journal
$ tune2fs -U da1f2ed0-60f6-aaaa-92fd-738701418523 /tmp/journal
$ sudo mke2fs -t ext4 -J device=/dev/loop0 /dev/loop1
$ dumpe2fs -h /tmp/dev | fgrep UUID
dumpe2fs 1.43-WIP (18-May-2014)
Filesystem UUID:          8a776be9-12eb-411f-8e88-b873575ecfb6
Journal UUID:             e3d02151-e776-4865-af25-aecb7291e8e5
$ sudo e2fsck /dev/vdc
e2fsck 1.43-WIP (18-May-2014)
External journal does not support this filesystem

/dev/loop1: ********** WARNING: Filesystem still has errors **********

Reported-by: Chin Tzung Cheng <chintzung@gmail.com>
Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>