FS-Cache: Root index definition

Add a description of the root index of the cache for later patches to make use
of.

The root index is owned by FS-Cache itself.  When a netfs requests caching
facilities, FS-Cache will, if one doesn't already exist, create an entry in
the root index with the key being the name of the netfs ("AFS" for example),
and the auxiliary data holding the index structure version supplied by the
netfs:

				     FSDEF
				       |
				 +-----------+
				 |           |
				NFS         AFS
			       [v=1]       [v=1]

If an entry with the appropriate name does already exist, the version is
compared.  If the version is different, the entire subtree from that entry
will be discarded and a new entry created.

The new entry will be an index, and a cookie referring to it will be passed to
the netfs.  This is then the root handle by which the netfs accesses the
cache.  It can create whatever objects it likes in that index, including
further indices.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>