udf: Fix extension of the last extent in the file

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>