udf: Fix preallocation discarding at indirect extent boundary

commit cfe4c1b25dd6d2f056afc00b7c98bcb3dd0b1fc3 upstream.

When preallocation extent is the first one in the extent block, the
code would corrupt extent tree header instead. Fix the problem and use
udf_delete_aext() for deleting extent to avoid some code duplication.

CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>