resize2fs: exit fix_sb_journal_backup early for external journal

resize2fs: exit fix_sb_journal_backup early for external journal

Resizing a filesystem with an external journal fails when it tries
to read inode 0:

# touch testfs
# truncate testfs 1342177280
# touch testjournal
# truncate testjournal 134217728
# mke2fs -O journal_dev testjournal
# losetup /dev/loop0 testjournal
# mkfs.ext4 -J device=/dev/loop0 testfs 127680
# resize2fs testfs
resize2fs 1.41.9 (22-Aug-2009)
Resizing the filesystem on testfs to 327680 (4k) blocks.
resize2fs: Illegal inode number while trying to resize testfs
Please run 'e2fsck -fy testfs' to fix the filesystem
after the aborted resize operation.

I think the right, simple thing to do is just bail out early
for an external journal here, as there are no backup blocks
to update.

Reported-by: mjevans1983@gmail.com
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>