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) <alexis.lothore@bootlin.com>
Fixes: f957c230 ("selftests/bpf: convert test_skb_cgroup_id_user to test_progs")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Reviewed-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241008231232.634047-1-tony.ambardar@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>