PCI: Replace zero-length array with flexible-array

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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

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