ext4: sync up block group descriptor with e2fsprogs.

This patch extends bg_itable_unused of ext4 group descriptor
from 16bit into 32bit. In order to add bg_itable_unused_hi into
struct ext4_group_desc, some extra fields which are already introduced into
e2fsprogs are also added in for consistency.

Signed-off-by: Coly Li <coyli@suse.de>
Cc: Andreas Dilger <adilger@clusterfs.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>