ext2/4: Add basic extent tree support
ext2/4: Add basic extent tree support

This allows booting from ext4 filesystems when EXT4_EXTENTS_FL is set in
inode.

Based on:
https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout

Signed-off-by: Alexey Kardashevskiy <ac1fc56530d7d037e1ea666f130daca22d72f964@ozlabs.ru>

qemu/SLOF,aik/SLOF,qemu/SLOF,aik/SLOF,stefanberger/SLOF-tpm,stefanberger/SLOF-tpm,qemu/SLOF,stefanberger/SLOF-tpm,aik/SLOF