ndisc: 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: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/ndisc.h

@@ -80,12 +80,12 @@ extern struct neigh_table nd_tbl;
 struct nd_msg {
         struct icmp6hdr	icmph;
         struct in6_addr	target;
-	__u8		opt[0];
+	__u8		opt[];
 };
 
 struct rs_msg {
 	struct icmp6hdr	icmph;
-	__u8		opt[0];
+	__u8		opt[];
 };
 
 struct ra_msg {
@@ -98,7 +98,7 @@ struct rd_msg {
 	struct icmp6hdr icmph;
 	struct in6_addr	target;
 	struct in6_addr	dest;
-	__u8		opt[0];
+	__u8		opt[];
 };
 
 struct nd_opt_hdr {