randstruct: Whitelist UNIXCB cast

This is another false positive in bad cast detection:

net/unix/af_unix.c: In function ‘unix_skb_scm_eq’:
net/unix/af_unix.c:1621:31: note: found mismatched rhs struct pointer types: ‘struct unix_skb_parms’ and ‘char’

  const struct unix_skb_parms *u = &UNIXCB(skb);
                               ^

UNIXCB is:

	#define UNIXCB(skb)     (*(struct unix_skb_parms *)&((skb)->cb))

And ->cb is:

	char                    cb[48] __aligned(8);

This is a rather crazy cast, but appears to be safe in the face of
randomization, so whitelist it in the plugin.

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: NKees Cook <keescook@chromium.org>

scripts/gcc-plugins/randomize_layout_plugin.c

@@ -46,6 +46,8 @@ struct whitelist_entry {
 };
 
 static const struct whitelist_entry whitelist[] = {
+	/* unix_skb_parms via UNIXCB() buffer */
+	{ "net/unix/af_unix.c", "unix_skb_parms", "char" },
 	/* walk struct security_hook_heads as an array of struct list_head */
 	{ "security/security.c", "list_head", "security_hook_heads" },
 	{ }