libext2fs: fix key extension in undo_write_tdb

libext2fs: fix key extension in undo_write_tdb

Support key extension if the tdb_data_size is an arbitrary integer
multiple of the channel's block size. Before, key extension was only
possible if the tdb_data_size and the channel's block size were
equal.
Note: a key, whose data is the result of a short read, will be
extended if the tdb_data_size and the channel's block size are equal
(that's what the old code did) (if tdb_data_size is an arbitrary
integer multiple (> 1) of the channel's block size, the key might
be extended as well (depending on the keysize)).

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