PCI: 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 as 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 76497732 ("cxgb3/l2t: Fix undefined behaviour")

Link: https://lore.kernel.org/r/20200507190544.GA15633@embeddedorSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci.c

@@ -1578,7 +1578,7 @@ EXPORT_SYMBOL(pci_restore_state);
 
 struct pci_saved_state {
 	u32 config_space[16];
-	struct pci_cap_saved_data cap[0];
+	struct pci_cap_saved_data cap[];
 };
 
 /**

include/linux/pci.h

@@ -279,7 +279,7 @@ struct pci_cap_saved_data {
 	u16		cap_nr;
 	bool		cap_extended;
 	unsigned int	size;
-	u32		data[0];
+	u32		data[];
 };
 
 struct pci_cap_saved_state {
@@ -532,7 +532,7 @@ struct pci_host_bridge {
 			resource_size_t start,
 			resource_size_t size,
 			resource_size_t align);
-	unsigned long	private[0] ____cacheline_aligned;
+	unsigned long	private[] ____cacheline_aligned;
 };
 
 #define	to_pci_host_bridge(n) container_of(n, struct pci_host_bridge, dev)