selftests/bpf: Fix sockopt_sk selftest

Commit f4e4534 ("net/netlink: fix NETLINK_LIST_MEMBERSHIPS length report")
fixed NETLINK_LIST_MEMBERSHIPS length report which caused
selftest sockopt_sk failure. The failure log looks like

  test_sockopt_sk:PASS:join_cgroup /sockopt_sk 0 nsec
  run_test:PASS:skel_load 0 nsec
  run_test:PASS:setsockopt_link 0 nsec
  run_test:PASS:getsockopt_link 0 nsec
  getsetsockopt:FAIL:Unexpected NETLINK_LIST_MEMBERSHIPS value unexpected Unexpected NETLINK_LIST_MEMBERSHIPS value: actual 8 != expected 4
  run_test:PASS:getsetsockopt 0 nsec
  #201     sockopt_sk:FAIL

In net/netlink/af_netlink.c, function netlink_getsockopt(), for NETLINK_LIST_MEMBERSHIPS,
nlk->ngroups equals to 36. Before Commit f4e4534, the optlen is calculated as
  ALIGN(nlk->ngroups / 8, sizeof(u32)) = 4
After that commit, the optlen is
  ALIGN(BITS_TO_BYTES(nlk->ngroups), sizeof(u32)) = 8

Fix the test by setting the expected optlen to be 8.

Fixes: f4e4534 ("net/netlink: fix NETLINK_LIST_MEMBERSHIPS length report")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230606172202.1606249-1-yhs@fb.com

Author: yonghong-song

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

@@ -209,7 +209,7 @@ static int getsetsockopt(void)
 			err, errno);
 		goto err;
 	}
-	ASSERT_EQ(optlen, 4, "Unexpected NETLINK_LIST_MEMBERSHIPS value");
+	ASSERT_EQ(optlen, 8, "Unexpected NETLINK_LIST_MEMBERSHIPS value");
 
 	free(big_buf);
 	close(fd);