sysctl.2: Use sizeof consistently

Use ``sizeof`` consistently through all the examples in the following
way:

- Use the name of the variable instead of its type as argument for
  ``sizeof``.

	Rationale:
	https://www.kernel.org/doc/html/v5.8/process/coding-style.html#allocating-memory

Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/sysctl.2

@@ -154,7 +154,7 @@ main(void)
     size_t osnamelth;
     int name[] = { CTL_KERN, KERN_OSTYPE };
 
-    memset(&args, 0, sizeof(struct __sysctl_args));
+    memset(&args, 0, sizeof(args));
     args.name = name;
     args.nlen = sizeof(name)/sizeof(name[0]);
     args.oldval = osname;