afs: Support the AFS dynamic root

Support the AFS dynamic root which is a pseudo-volume that doesn't connect
to any server resource, but rather is just a root directory that
dynamically creates mountpoint directories where the name of such a
directory is the name of the cell.

Such a mount can be created thus:

	mount -t afs none /afs -o dyn

Dynamic root superblocks aren't shared except by bind mounts and
propagation.  Cell root volumes can then be mounted by referring to them by
name, e.g.:

	ls /afs/grand.central.org/
	ls /afs/.grand.central.org/

The kernel will upcall to consult the DNS if the address wasn't supplied
directly.

Signed-off-by: David Howells <dhowells@redhat.com>

Documentation/filesystems/afs.txt

@@ -7,6 +7,7 @@ Contents:
  - Overview.
  - Usage.
  - Mountpoints.
+ - Dynamic root.
  - Proc filesystem.
  - The cell database.
  - Security.
@@ -127,6 +128,22 @@ mounted on /afs in one go by doing:
 	umount /afs
 
 
+============
+DYNAMIC ROOT
+============
+
+A mount option is available to create a serverless mount that is only usable
+for dynamic lookup.  Creating such a mount can be done by, for example:
+
+	mount -t afs none /afs -o dyn
+
+This creates a mount that just has an empty directory at the root.  Attempting
+to look up a name in this directory will cause a mountpoint to be created that
+looks up a cell of the same name, for example:
+
+	ls /afs/grand.central.org/
+
+
 ===============
 PROC FILESYSTEM
 ===============