ceph: renew auth tickets before they expire

We were only requesting renewal after our tickets expire; do so before
that.  Most of the low-level logic for this was already there; just use
it.

Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/auth.h

@@ -23,6 +23,12 @@ struct ceph_auth_client_ops {
 	 */
 	int (*is_authenticated)(struct ceph_auth_client *ac);
 
+	/*
+	 * true if we should (re)authenticate, e.g., when our tickets
+	 * are getting old and crusty.
+	 */
+	int (*should_authenticate)(struct ceph_auth_client *ac);
+
 	/*
 	 * build requests and process replies during monitor
 	 * handshake.  if handle_reply returns -EAGAIN, we build