libext2fs: fix offset code in undo_write_tdb

libext2fs: fix offset code in undo_write_tdb

The old code has some issues, for example, when backing up fs block 0
(can be reproduced via "mke2fs -z undo -b 1024 -E offset=1024 out 1024"):
* backing_blk_num is set to ULLONG_MAX instead of 0
* data is read from the beginning of the file instead of offset 1024
* data_ptr is set to read_ptr - 1024 ("invalid" address)

Hence, the wrong fs block is associated with the wrong data.
For details, see also commit 76da764639cbfcc998f13c263a11a4601bcb9961.

Signed-off-by: Marcus Huewe <suse-tux@gmx.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>