ext4: Do not crash when trying to grow a directory using extents

The following command crashes u-boot:
./sandbox/u-boot -c 'i=0; host bind 0 ./sandbox/test/fs/3GB.ext4.img ;
  while test $i -lt 200 ; do echo $i; setexpr i $i + 1;
  ext4write host 0 0 /foobar${i} 0; done'

Previously, the code updated the direct_block even for extents, and
fortunately crashed before pushing garbage to the disk.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>