This patch allows forward compatibility with future filesystems which
are dynamically grown by using an alternate algorithm for storing the
block group descriptors.  It's also a bit more efficient, in that it
uses just a little bit less disk space.  Currently, the ext2 filesystem
format requires either relocating the inode table, or reserving space in
before doing the on-line resize.  The new scheme, which is documented in
"Planned Extensions to the Ext2/3 Filesystem", by Stephen Tweedie and I 
(see: http://e2fsprogs.sourceforge.net/extensions-ext23)