NFSD: Add an nfsd_cb_probe tracepoint

Record a tracepoint event when the server performs a callback
probe. This event can be enabled as a group with other nfsd_cb
tracepoints.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4callback.c

@@ -998,6 +998,7 @@ static const struct rpc_call_ops nfsd4_cb_probe_ops = {
  */
 void nfsd4_probe_callback(struct nfs4_client *clp)
 {
+	trace_nfsd_cb_probe(clp);
 	nfsd4_mark_cb_state(clp, NFSD4_CB_UNKNOWN);
 	set_bit(NFSD4_CLIENT_CB_UPDATE, &clp->cl_flags);
 	nfsd4_run_cb(&clp->cl_cb_null);

fs/nfsd/trace.h

@@ -910,6 +910,7 @@ DEFINE_EVENT(nfsd_cb_class, nfsd_cb_##name,		\
 	TP_ARGS(clp))
 
 DEFINE_NFSD_CB_EVENT(state);
+DEFINE_NFSD_CB_EVENT(probe);
 DEFINE_NFSD_CB_EVENT(lost);
 DEFINE_NFSD_CB_EVENT(shutdown);