selftests: net: af_unix: Fix a build error of unix_connect.c.

This patch fixes a build error reported in the link. [0]

unix_connect.c: In function ‘unix_connect_test’:
unix_connect.c:115:55: error: expected identifier before ‘(’ token
#define offsetof(type, member) ((size_t)&((type *)0)->(member))
^
unix_connect.c:128:12: note: in expansion of macro ‘offsetof’
addrlen = offsetof(struct sockaddr_un, sun_path) + variant->len;
^~~~~~~~

We can fix this by removing () around member, but checkpatch will complain
about it, and the root cause of the build failure is that I followed the
warning and fixed this in the v2 -> v3 change of the blamed commit. [1]

CHECK: Macro argument 'member' may be better as '(member)' to avoid precedence issues
#33: FILE: tools/testing/selftests/net/af_unix/unix_connect.c:115:
+#define offsetof(type, member) ((size_t)&((type *)0)->member)

To avoid this warning, let's use offsetof() defined in stddef.h instead.

[0]: https://lore.kernel.org/linux-mm/[e-mail]/
[1]: https://lore.kernel.org/netdev/[e-mail]/

Fixes: e95ab1d85289 ("selftests: net: af_unix: Test connect() with different netns.")
Reported-by: kernel test robot <[e-mail]>
Suggested-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/af_unix/unix_connect.c

···
 #define _GNU_SOURCE
 #include <sched.h>
 
+#include <stddef.h>
 #include <stdio.h>
 #include <unistd.h>
 
···
 	if (variant->sun_path[0])
 		remove("test");
 }
-
-#define offsetof(type, member) ((size_t)&((type *)0)->(member))
 
 TEST_F(unix_connect, test)
 {