tst-cancel4-common.c: fix calling socketpair

PF_UNIX was actually never intended to be passed as protocol parameter to
socket() calls: it is a protocol family, not a protocol.  It happens that
Linux introduced accepting it during its 2.0 development, but it shouldn't.
OpenBSD kernels accept it as well, but FreeBSD and NetBSD rightfully do not.
GNU/Hurd does not either.

* nptl/tst-cancel4-common.c (do_test): Pass 0 instead of PF_UNIX as
protocol.