afs: Add some more info to /proc/net/afs/servers

In /proc/net/afs/servers, show the cell name and the last error for each
address in the server's list.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org