gve: Replace zero-length array with flexible-array member

There is a regular need in the kernel to provide a way to declare having a
dynamically sized set of trailing elements in a structure.  Kernel code
should always use “flexible array members”[1] for these cases.  The
older style of one-element or zero-length arrays should no longer be
used[2].

Refactor the code according to the use of a flexible-array member in
struct gve_stats_report, instead of a zero-length array, and use the
struct_size() helper to calculate the size for the resource allocation.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arrays



Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>