afs: Fix missing cell comparison in afs_test_super()

Fix missing cell comparison in afs_test_super().  Without this, any pair
volumes that have the same volume ID will share a superblock, no matter the
cell, unless they're in different network namespaces.

Normally, most users will only deal with a single cell and so they won't
see this.  Even if they do look into a second cell, they won't see a
problem unless they happen to hit a volume with the same ID as one they've
already got mounted.

Before the patch:

    # ls /afs/grand.central.org/archive
    linuxdev/  mailman/  moin/  mysql/  pipermail/  stage/  twiki/
    # ls /afs/kth.se/
    linuxdev/  mailman/  moin/  mysql/  pipermail/  stage/  twiki/
    # cat /proc/mounts | grep afs
    none /afs afs rw,relatime,dyn,autocell 0 0
    #grand.central.org:root.cell /afs/grand.central.org afs ro,relatime 0 0
    #grand.central.org:root.archive /afs/grand.central.org/archive afs ro,relatime 0 0
    #grand.central.org:root.archive /afs/kth.se afs ro,relatime 0 0

After the patch:

    # ls /afs/grand.central.org/archive
    linuxdev/  mailman/  moin/  mysql/  pipermail/  stage/  twiki/
    # ls /afs/kth.se/
    admin/        common/  install/  OldFiles/  service/  system/
    bakrestores/  home/    misc/     pkg/       src/      wsadmin/
    # cat /proc/mounts | grep afs
    none /afs afs rw,relatime,dyn,autocell 0 0
    #grand.central.org:root.cell /afs/grand.central.org afs ro,relatime 0 0
    #grand.central.org:root.archive /afs/grand.central.org/archive afs ro,relatime 0 0
    #kth.se:root.cell /afs/kth.se afs ro,relatime 0 0

Fixes: ^1da177e4 ("Linux-2.6.12-rc2")
Reported-by: Carsten Jacobi <jacobi@de.ibm.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Marc Dionne <marc.dionne@auristor.com>
Tested-by: Jonathan Billings <jsbillings@jsbillings.org>
cc: Todd DeSantis <atd@us.ibm.com>