Don't follow smbnode n_parent pointer when NREFPARENT flag is not set

in smb_fphelp(): the parent vnode may have already been recycled
since we don't hold a reference to it. Fixes a panic when rebooting
with mdconfig -t vnode devices referring to vnodes on a smbfs mount.

sys/fs/smbfs/smbfs_subr.c

@@ -270,6 +270,8 @@ smb_fphelp(struct mbchain *mbp, struct smb_vc *vcp, struct smbnode *np,
 			return ENAMETOOLONG;
 		}
 		*npp++ = np;
+		if ((np->n_flag & NREFPARENT) == 0)
+			break;
 		np = VTOSMB(np->n_parent);
 	}
 /*	if (i == 0)