vmbus: Replace zero-length array with flexible-array

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]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

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 7649773 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200507185323.GA14416@embeddedor
Signed-off-by: Wei Liu <wei.liu@kernel.org>

include/linux/hyperv.h

@@ -117,7 +117,7 @@ struct hv_ring_buffer {
 	 * Ring data starts here + RingDataStartOffset
 	 * !!! DO NOT place any fields below this !!!
 	 */
-	u8 buffer[0];
+	u8 buffer[];
 } __packed;
 
 struct hv_ring_buffer_info {
@@ -313,7 +313,7 @@ struct vmadd_remove_transfer_page_set {
 struct gpa_range {
 	u32 byte_count;
 	u32 byte_offset;
-	u64 pfn_array[0];
+	u64 pfn_array[];
 };
 
 /*
@@ -563,15 +563,15 @@ struct vmbus_channel_gpadl_header {
 	u32 gpadl;
 	u16 range_buflen;
 	u16 rangecount;
-	struct gpa_range range[0];
+	struct gpa_range range[];
 } __packed;
 
 /* This is the followup packet that contains more PFNs. */
 struct vmbus_channel_gpadl_body {
 	struct vmbus_channel_message_header header;
 	u32 msgnumber;
 	u32 gpadl;
-	u64 pfn[0];
+	u64 pfn[];
 } __packed;
 
 struct vmbus_channel_gpadl_created {
@@ -679,7 +679,7 @@ struct vmbus_channel_msginfo {
 	 * The channel message that goes out on the "wire".
 	 * It will contain at minimum the VMBUS_CHANNEL_MESSAGE_HEADER header
 	 */
-	unsigned char msg[0];
+	unsigned char msg[];
 };
 
 struct vmbus_close_msg {