When extending the last extent in the file within the last block, we
wrongly computed the length of the last extent. This is mostly a
cosmetical problem since the extent does not contain any data and the
length will be fixed up by following operations but still.

Fixes: 1f3868f0 ("udf: Fix extending file within last block")
Signed-off-by: Jan Kara <jack@suse.cz>