iplboot: add ext4 support

For a read only filesystem, like iplboot, the only real additions for
ext4 are allowing for extent based inodes and a variable group size.

The current block transformation scheme simply goes from filesystem
block offset to absolute partition block employing a caching scheme
for the indirect inodes.  We can follow a similar scheme for the
extent tree based on depth, relying on the fact that linear block
loading will optimally keep the cache at a given depth until it's not
needed.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Helge Deller <deller@gmx.de>