SUNRPC: AUTH_SYS "machine creds" shouldn't use negative valued uid/gid

Apparently this causes Solaris 10 servers to refuse our NFSv4 SETCLIENTID
calls. Fall back to root creds for now, since most servers that care are
very likely to have root squashing enabled.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>