afs: Note the cell in the superblock info also

Keep a reference to the cell in the superblock info structure in addition
to the volume and net pointers.  This will make it easier to clean up in a
future patch in which afs_put_volume() will need the cell pointer.

Whilst we're at it, make the cell and volume getting functions return a
pointer to the object got to make the call sites look neater.

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