PCI: hv: 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: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Dexuan Cui <decui@microsoft.com>

drivers/pci/controller/pci-hyperv.c

@@ -283,7 +283,7 @@ struct pci_packet {
 				int resp_packet_size);
 	void *compl_ctxt;
 
-	struct pci_message message[0];
+	struct pci_message message[];
 };
 
 /*
@@ -319,13 +319,13 @@ struct pci_bus_d0_entry {
 struct pci_bus_relations {
 	struct pci_incoming_message incoming;
 	u32 device_count;
-	struct pci_function_description func[0];
+	struct pci_function_description func[];
 } __packed;
 
 struct pci_bus_relations2 {
 	struct pci_incoming_message incoming;
 	u32 device_count;
-	struct pci_function_description2 func[0];
+	struct pci_function_description2 func[];
 } __packed;
 
 struct pci_q_res_req_response {
@@ -551,7 +551,7 @@ struct hv_pcidev_description {
 struct hv_dr_state {
 	struct list_head list_entry;
 	u32 device_count;
-	struct hv_pcidev_description func[0];
+	struct hv_pcidev_description func[];
 };
 
 enum hv_pcichild_state {