selinux: Annotate struct sidtab_str_cache with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct sidtab_str_cache.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Paul Moore <paul@paul-moore.com>
Cc: Stephen Smalley <stephen.smalley.work@gmail.com>
Cc: Eric Paris <eparis@parisplace.org>
Cc: Ondrej Mosnacek <omosnace@redhat.com>
Cc: selinux@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/sidtab.c

@@ -25,7 +25,7 @@ struct sidtab_str_cache {
 	struct list_head lru_member;
 	struct sidtab_entry *parent;
 	u32 len;
-	char str[];
+	char str[] __counted_by(len);
 };
 
 #define index_to_sid(index) ((index) + SECINITSID_NUM + 1)