afs: Don't get epoch from a server because it may be ambiguous

Don't get the epoch from a server, particularly one that we're looking up
by UUID, as UUIDs may be ambiguous and may map to more than one server - so
we can't draw any conclusions from it.

Reported-by: Jeffrey Altman <jaltman@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>