net: ipv6: mld: 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/mld.h

@@ -24,12 +24,12 @@ struct mld2_grec {
 	__u8		grec_auxwords;
 	__be16		grec_nsrcs;
 	struct in6_addr	grec_mca;
-	struct in6_addr	grec_src[0];
+	struct in6_addr	grec_src[];
 };
 
 struct mld2_report {
 	struct icmp6hdr		mld2r_hdr;
-	struct mld2_grec	mld2r_grec[0];
+	struct mld2_grec	mld2r_grec[];
 };
 
 #define mld2r_type		mld2r_hdr.icmp6_type
@@ -55,7 +55,7 @@ struct mld2_query {
 #endif
 	__u8			mld2q_qqic;
 	__be16			mld2q_nsrcs;
-	struct in6_addr		mld2q_srcs[0];
+	struct in6_addr		mld2q_srcs[];
 };
 
 #define mld2q_type		mld2q_hdr.icmp6_type