arcnet: 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/uapi/linux/if_arcnet.h

@@ -60,7 +60,7 @@ struct arc_rfc1201 {
 	__u8  proto;		/* protocol ID field - varies		*/
 	__u8  split_flag;	/* for use with split packets		*/
 	__be16   sequence;	/* sequence number			*/
-	__u8  payload[0];	/* space remaining in packet (504 bytes)*/
+	__u8  payload[];	/* space remaining in packet (504 bytes)*/
 };
 #define RFC1201_HDR_SIZE 4
 
@@ -69,7 +69,7 @@ struct arc_rfc1201 {
  */
 struct arc_rfc1051 {
 	__u8 proto;		/* ARC_P_RFC1051_ARP/RFC1051_IP	*/
-	__u8 payload[0];	/* 507 bytes			*/
+	__u8 payload[];	/* 507 bytes			*/
 };
 #define RFC1051_HDR_SIZE 1
 
@@ -80,7 +80,7 @@ struct arc_rfc1051 {
 struct arc_eth_encap {
 	__u8 proto;		/* Always ARC_P_ETHER			*/
 	struct ethhdr eth;	/* standard ethernet header (yuck!)	*/
-	__u8 payload[0];	/* 493 bytes				*/
+	__u8 payload[];	/* 493 bytes				*/
 };
 #define ETH_ENCAP_HDR_SIZE 14