journal: use consts instead of 1024 and add helper for journal with 1k blocksize

Use EXT2_MIN_BLOCK_SIZE, JFS_MIN_JOURNAL_BLOCKS, SUPERBLOCK_SIZE, and
SUPERBLOCK_OFFSET instead of hardcoded 1024 when it is okay, and also
add a helper ext2fs_journal_sb_start() that will return start of
journal sb with special case for fs with 1k block size.

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>
tune2fs: remove_journal_device(): use the correct block to find jsb

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>
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>
tune2fs: update journal users while updating fs UUID (with external journal)

When we have fs with external journal device, and updating it's UUID, we
should update UUID in users list for that external journal device.

Before:
$ tune2fs -U clear /tmp/dev
tune2fs 1.42.10 (18-May-2014)
$ dumpe2fs /tmp/dev | fgrep UUID
dumpe2fs 1.42.10 (18-May-2014)
Filesystem UUID:          <none>
Journal UUID:             da1f2ed0-60f6-aaaa-92fd-738701418523
$ dumpe2fs /tmp/journal | fgrep users -A10
dumpe2fs 1.42.10 (18-May-2014)
Journal number of users:  2
Journal users:            0707762d-638e-4bc6-944e-ae8ee7a3359e
                          0ad849df-1041-4f0a-b1c1-2f949d6a1e37

After:
$ sudo tune2fs -U clear /tmp/dev
tune2fs 1.43-WIP (18-May-2014)
$ dumpe2fs /tmp/dev | fgrep UUID
dumpe2fs 1.42.10 (18-May-2014)
Filesystem UUID:          <none>
Journal UUID:             da1f2ed0-60f6-aaaa-92fd-738701418523
$ dumpe2fs /tmp/journal | fgrep users -A10
dumpe2fs 1.42.10 (18-May-2014)
Journal number of users:  2
Journal users:            0707762d-638e-4bc6-944e-ae8ee7a3359e
                          00000000-0000-0000-0000-000000000000

Also add some consts to avoid *magic numbers*:
- UUID_STR_SIZE
- UUID_SIZE
- JFS_USERS_MAX
- JFS_USERS_SIZE

Proposed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>