selftests/bpf: Fix error compiling cgroup_ancestor.c with musl libc

Existing code calls connect() with a 'struct sockaddr_in6 *' argument
where a 'struct sockaddr *' argument is declared, yielding compile errors
when building for mips64el/musl-libc:

In file included from cgroup_ancestor.c:3:
cgroup_ancestor.c: In function 'send_datagram':
cgroup_ancestor.c:38:38: error: passing argument 2 of 'connect' from incompatible pointer type [-Werror=incompatible-pointer-types]
38 | if (!ASSERT_OK(connect(sock, &addr, sizeof(addr)), "connect")) {
| ^~~~~
| |
| struct sockaddr_in6 *
./test_progs.h:343:29: note: in definition of macro 'ASSERT_OK'
343 | long long ___res = (res); \
| ^~~
In file included from .../netinet/in.h:10,
from .../arpa/inet.h:9,
from ./test_progs.h:17:
.../sys/socket.h:386:19: note: expected 'const struct sockaddr *' but argument is of type 'struct sockaddr_in6 *'
386 | int connect (int, const struct sockaddr *, socklen_t);
| ^~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

This only compiles because of a glibc extension allowing declaration of the
argument as a "transparent union" which includes both types above.

Explicitly cast the argument to allow compiling for both musl and glibc.

Cc: Alexis Lothoré (eBPF Foundation) <[e-mail]>
Fixes: f957c230e173 ("selftests/bpf: convert test_skb_cgroup_id_user to test_progs")
Signed-off-by: Tony Ambardar <[e-mail]>
Reviewed-by: Alexis Lothoré <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 	if (!ASSERT_OK_FD(sock, "create socket"))
 		return sock;
 
-	if (!ASSERT_OK(connect(sock, &addr, sizeof(addr)), "connect")) {
+	if (!ASSERT_OK(connect(sock, (struct sockaddr *)&addr, sizeof(addr)), "connect")) {
 		close(sock);
 		return -1;
 	}