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

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

  bpf_iter_setsockopt.c: In function 'get_local_port':
  bpf_iter_setsockopt.c:98:30: error: passing argument 2 of 'getsockname' from incompatible pointer type [-Werror=incompatible-pointer-types]
     98 |         if (!getsockname(fd, &addr, &addrlen))
        |                              ^~~~~
        |                              |
        |                              struct sockaddr_in6 *
  In file included from .../netinet/in.h:10,
                   from .../arpa/inet.h:9,
                   from ./test_progs.h:17,
                   from bpf_iter_setsockopt.c:5:
  .../sys/socket.h:391:23: note: expected 'struct sockaddr * restrict' but argument is of type 'struct sockaddr_in6 *'
    391 | int getsockname (int, struct sockaddr *__restrict, socklen_t *__restrict);
        |                       ^
  cc1: all warnings being treated as errors

This compiled under glibc only because the argument is declared to be a
"funky" transparent union which includes both types above. Explicitly cast
the argument to allow compiling for both musl and glibc.

Fixes: eed92afdd1 ("bpf: selftest: Test batching and bpf_(get|set)sockopt in bpf tcp iter")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Geliang Tang <geliang@kernel.org>
Link: https://lore.kernel.org/bpf/f41def0f17b27a23b1709080e4e3f37f4cc11ca9.1721713597.git.tony.ambardar@gmail.com

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

@@ -95,7 +95,7 @@ static unsigned short get_local_port(int fd)
 	struct sockaddr_in6 addr;
 	socklen_t addrlen = sizeof(addr);
 
-	if (!getsockname(fd, &addr, &addrlen))
+	if (!getsockname(fd, (struct sockaddr *)&addr, &addrlen))
 		return ntohs(addr.sin6_port);
 
 	return 0;