net: dccp: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

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
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/dccp.h

@@ -198,7 +198,7 @@ enum dccp_role {
 
 struct dccp_service_list {
 	__u32	dccpsl_nr;
-	__be32	dccpsl_list[0];
+	__be32	dccpsl_list[];
 };
 
 #define DCCP_SERVICE_INVALID_VALUE htonl((__u32)-1)

net/dccp/ccid.h

@@ -95,7 +95,7 @@ void ccid_cleanup_builtins(void);
 
 struct ccid {
 	struct ccid_operations *ccid_ops;
-	char		       ccid_priv[0];
+	char		       ccid_priv[];
 };
 
 static inline void *ccid_priv(const struct ccid *ccid)