EXT2FS: fix inode size for ext2fs rev#0

extfs.c assumes that there is always a valid inode_size field in the
superblock. But this is not true for ext2fs rev 0. Such ext2fs images
are for instance generated by genext2fs. Symptoms on ARM machines are
messages like: "raise: Signal # 8 caught"; on PowerPC "ext2ls" will
print nothing.
This fix checks for rev 0 and uses then 128 bytes as inode size.

Signed-off-by: Michael Brandt <Michael.Brandt@emsyso.de>
Tested on: TQM5200S
Tested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Wolfgang Denk <wd@denx.de>