fs/ext4: Initialize group descriptor size for revision level 0 filesystems

genext2fs creates revision level 0 filesystems, which are not readable
by u-boot due to the initialized group descriptor size field.
f798b1dd

Reported-by: Kever Yang <kever.yang@rock-chips.com>
Reported-by:  <FrostyBytes@protonmail.com>
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Tested-by: Kever Yang <kever.yang@rock-chips.com>