net: mpls: 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/mpls_iptunnel.h

@@ -11,7 +11,7 @@ struct mpls_iptunnel_encap {
 	u8	ttl_propagate;
 	u8	default_ttl;
 	u8	reserved1;
-	u32	label[0];
+	u32	label[];
 };
 
 static inline struct mpls_iptunnel_encap *mpls_lwtunnel_encap(struct lwtunnel_state *lwtstate)

net/mpls/internal.h

@@ -98,7 +98,7 @@ struct mpls_nh { /* next hop label forwarding entry */
 	u8			nh_via_table;
 	u8			nh_reserved1;
 
-	u32			nh_label[0];
+	u32			nh_label[];
 };
 
 /* offset of via from beginning of mpls_nh */
@@ -154,7 +154,7 @@ struct mpls_route { /* next hop label forwarding entry */
 	u8			rt_nh_size;
 	u8			rt_via_offset;
 	u8			rt_reserved1;
-	struct mpls_nh		rt_nh[0];
+	struct mpls_nh		rt_nh[];
 };
 
 #define for_nexthops(rt) {						\