afs: support the CB.ProbeUuid RPC op

Add support for the CB.ProbeUuid cache manager RPC op.  This allows a modern
OpenAFS server to quickly ask if the client has been rebooted.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>