xen: 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>
Link: https://lore.kernel.org/r/20200226212612.GA4663@embeddedorReviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xen-pciback/pciback.h

@@ -52,7 +52,7 @@ struct xen_pcibk_dev_data {
 	unsigned int ack_intr:1; /* .. and ACK-ing */
 	unsigned long handled;
 	unsigned int irq; /* Saved in case device transitions to MSI/MSI-X */
-	char irq_name[0]; /* xen-pcibk[000:04:00.0] */
+	char irq_name[]; /* xen-pcibk[000:04:00.0] */
 };
 
 /* Used by XenBus and xen_pcibk_ops.c */

include/xen/interface/io/tpmif.h

@@ -46,7 +46,7 @@ struct vtpm_shared_page {
 	uint8_t pad;
 
 	uint8_t nr_extra_pages;  /* extra pages for long packets; may be zero */
-	uint32_t extra_pages[0]; /* grant IDs; length in nr_extra_pages */
+	uint32_t extra_pages[]; /* grant IDs; length in nr_extra_pages */
 };
 
 #endif