net: sched: cls_u32: Use flexible-array member

Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
presence of a "variable length array":

struct something {
    int length;
    u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

There is also 0-byte arrays. Both cases pose confusion for things like
sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
to declare variable-length types such as the one above is a flexible array
member[2] which need to be the last member of a structure and empty-sized:

struct something {
        int stuff;
        u8 data[];
};

Also, by making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
unadvertenly introduced[3] to the codebase from now on.

[1] KSPP#21
[2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[3] commit 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

net/sched/cls_u32.c

@@ -79,7 +79,7 @@ struct tc_u_hnode {
 	/* The 'ht' field MUST be the last field in structure to allow for
 	 * more entries allocated at end of structure.
 	 */
-	struct tc_u_knode __rcu	*ht[1];
+	struct tc_u_knode __rcu	*ht[];
 };
 
 struct tc_u_common {