Fix two bugs in DTrace tracing of accesscache and attrcache load events:

- Trace non-error loads into the access cache once, not zero times or
  twice.
- Sometimes attr cache loads fail due to a race, in which case they are
  aborted leading to an invalidation; in this case, trace only the flush,
  not a load.

MFC after:	1 month
Sponsored by:	Google, Inc.

sys/nfsclient/nfs_kdtrace.h

@@ -64,8 +64,7 @@ extern uint32_t nfsclient_accesscache_load_done_id;
 } while (0)
 
 #define	KDTRACE_NFS_ACCESSCACHE_LOAD_DONE(vp, uid, rmode, error) do {	\
-	if (error && dtrace_nfsclient_accesscache_load_done_probe !=	\
-	    NULL)							\
+	if (dtrace_nfsclient_accesscache_load_done_probe != NULL)	\
 		(dtrace_nfsclient_accesscache_load_done_probe)(		\
 		    nfsclient_accesscache_load_done_id, (vp), (uid),	\
 		    (rmode), (error));					\

sys/nfsclient/nfs_subs.c

@@ -755,12 +755,16 @@ nfs_loadattrcache(struct vnode **vpp, struct mbuf **mdp, caddr_t *dposp,
 				vaper->va_mtime = np->n_mtim;
 		}
 	}
+
+#ifdef KDTRACE_HOOKS
+	if (np->n_attrstamp != 0)
+		KDTRACE_NFS_ATTRCACHE_LOAD_DONE(vp, &np->n_vattr, 0);
+#endif
 	mtx_unlock(&np->n_mtx);
 out:
-#ifdef KDRACE_HOOKS
-	if (np != NULL)
-		KDTRACE_NFS_ATTRCACHE_LOAD_DONE(vp, error == 0 ? &np->n_vattr
-		    : NULL, error);
+#ifdef KDTRACE_HOOKS
+	if (error)
+		KDTRACE_NFS_ATTRCACHE_LOAD_DONE(vp, NULL, error);
 #endif
 	return (error);
 }

sys/nfsclient/nfs_vnops.c

@@ -339,7 +339,12 @@ nfs3_access_otw(struct vnode *vp, int wmode, struct thread *td,
 	}
 	m_freem(mrep);
 nfsmout:
-	KDTRACE_NFS_ACCESSCACHE_LOAD_DONE(vp, cred->cr_uid, 0, error);
+#ifdef KDTRACE_HOOKS
+	if (error) {
+		KDTRACE_NFS_ACCESSCACHE_LOAD_DONE(vp, cred->cr_uid, 0,
+		    error);
+	}
+#endif
 	return (error);
 }