ext2fs: Update Block Group Descriptor struct.
ext2fs: Update Block Group Descriptor struct.

Uncover some, previously reserved, fields that are used by Ext4.
These are currently unused but it is good to have them for future
reference.

Reviewed by:	bde
MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase