net: sched: 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/net/pkt_sched.h

@@ -181,7 +181,7 @@ struct tc_taprio_qopt_offload {
 	u64 cycle_time_extension;
 
 	size_t num_entries;
-	struct tc_taprio_sched_entry entries[0];
+	struct tc_taprio_sched_entry entries[];
 };
 
 /* Reference counting */

net/sched/em_ipt.c

@@ -22,7 +22,7 @@ struct em_ipt_match {
 	const struct xt_match *match;
 	u32 hook;
 	u8 nfproto;
-	u8 match_data[0] __aligned(8);
+	u8 match_data[] __aligned(8);
 };
 
 struct em_ipt_xt_match {

net/sched/em_nbyte.c

@@ -16,7 +16,7 @@
 
 struct nbyte_data {
 	struct tcf_em_nbyte	hdr;
-	char			pattern[0];
+	char			pattern[];
 };
 
 static int em_nbyte_change(struct net *net, void *data, int data_len,

net/sched/sch_atm.c

@@ -58,7 +58,7 @@ struct atm_flow_data {
 	struct atm_flow_data	*excess;	/* flow for excess traffic;
 						   NULL to set CLP instead */
 	int			hdr_len;
-	unsigned char		hdr[0];		/* header data; MUST BE LAST */
+	unsigned char		hdr[];		/* header data; MUST BE LAST */
 };
 
 struct atm_qdisc_data {

net/sched/sch_netem.c

@@ -66,7 +66,7 @@
 
 struct disttable {
 	u32  size;
-	s16 table[0];
+	s16 table[];
 };
 
 struct netem_sched_data {