Bluetooth: 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 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/bluetooth/l2cap.h

@@ -299,14 +299,14 @@ struct l2cap_conn_rsp {
 struct l2cap_conf_req {
 	__le16     dcid;
 	__le16     flags;
-	__u8       data[0];
+	__u8       data[];
 } __packed;
 
 struct l2cap_conf_rsp {
 	__le16     scid;
 	__le16     flags;
 	__le16     result;
-	__u8       data[0];
+	__u8       data[];
 } __packed;
 
 #define L2CAP_CONF_SUCCESS	0x0000
@@ -322,7 +322,7 @@ struct l2cap_conf_rsp {
 struct l2cap_conf_opt {
 	__u8       type;
 	__u8       len;
-	__u8       val[0];
+	__u8       val[];
 } __packed;
 #define L2CAP_CONF_OPT_SIZE	2
 
@@ -392,7 +392,7 @@ struct l2cap_info_req {
 struct l2cap_info_rsp {
 	__le16      type;
 	__le16      result;
-	__u8        data[0];
+	__u8        data[];
 } __packed;
 
 struct l2cap_create_chan_req {