drop_monitor: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

---
 include/uapi/linux/net_dropmon.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 66048cc5d7b3..67e31f329190 100644
@@ -29,12 +29,12 @@ struct net_dm_config_entry {
 
 struct net_dm_config_msg {
 	__u32 entries;
-	struct net_dm_config_entry options[0];
+	struct net_dm_config_entry options[];
 };
 
 struct net_dm_alert_msg {
 	__u32 entries;
-	struct net_dm_drop_point points[0];
+	struct net_dm_drop_point points[];
 };
 
 struct net_dm_user_msg {

2.25.0

From: "Gustavo A. R. Silva" <[email protected]>
Date: Mon, 2 Mar 2020 06:02:09 -0600

Applied.