afs: Fix to actually set AFS_SERVER_FL_HAVE_EPOCH

AFS keeps track of the epoch value from the rxrpc protocol to note (a) when
a fileserver appears to have restarted and (b) when different endpoints of
a fileserver do not appear to be associated with the same fileserver
(ie. all probes back from a fileserver from all of its interfaces should
carry the same epoch).

However, the AFS_SERVER_FL_HAVE_EPOCH flag that indicates that we've
received the server's epoch is never set, though it is used.

Fix this to set the flag when we first receive an epoch value from a probe
sent to the filesystem client from the fileserver.

Fixes: 3bf0fb6f ("afs: Probe multiple fileservers simultaneously")
Signed-off-by: David Howells <dhowells@redhat.com>