selftests/bpf: Add pid limit for mptcpify prog

In order to prevent mptcpify prog from affecting the running results
of other BPF tests, a pid limit was added to restrict it from only
modifying its own program.

Suggested-by: Martin KaFai Lau <[e-mail]>
Signed-off-by: Geliang Tang <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/898[e-mail]

tools/testing/selftests/bpf/prog_tests/mptcp.c

···
 	if (!ASSERT_OK_PTR(mptcpify_skel, "skel_open_load"))
 		return libbpf_get_error(mptcpify_skel);
 
+	mptcpify_skel->bss->pid = getpid();
+
 	err = mptcpify__attach(mptcpify_skel);
 	if (!ASSERT_OK(err, "skel_attach"))
 		goto out;

tools/testing/selftests/bpf/progs/mptcpify.c

···
 #include "bpf_tracing_net.h"
 
 char _license[] SEC("license") = "GPL";
+int pid;
 
 SEC("fmod_ret/update_socket_protocol")
 int BPF_PROG(mptcpify, int family, int type, int protocol)
 {
+	if (bpf_get_current_pid_tgid() >> 32 != pid)
+		return protocol;
+
 	if ((family == AF_INET || family == AF_INET6) &&
 	    type == SOCK_STREAM &&
 	    (!protocol || protocol == IPPROTO_TCP)) {