(now uses struct block_device * to index devices, and uses hash.h for hash function)

This patch creates a meta block cache which is utilized by the ext3 and
ext2 extended attribute patch (patches 2 and 3, respectively).  This
cache allows directory blocks to be indexed by multiple keys.  In the
case of the extended attribute patches, it is used to look up blocks by
both the block number and by the hash of the extended attributes.  This
is extremely important to allow the sharing of acl's when stored as
extended attributes.  Otherwise every single file would require its own,
separate, one block overhead to store then ACL, even though there might
be a large number of files that have the same ACL.