bpf: Replace 0-length arrays with flexible arrays

Zero-length arrays are deprecated [1]. Replace struct bpf_array's union
of 0-length arrays with flexible arrays. Detected with GCC 13, by using
-fstrict-flex-arrays=3:

  arch/x86/net/bpf_jit_comp.c: In function 'bpf_tail_call_direct_fixup':
  arch/x86/net/bpf_jit_comp.c:606:37: warning: array subscript <unknown> is outside array bounds of 'void *[0]' [-Warray-bounds=]
    606 |                 target = array->ptrs[poke->tail_call.key];
        |                          ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
  In file included from include/linux/filter.h:9,
                   from arch/x86/net/bpf_jit_comp.c:9:
  include/linux/bpf.h:1527:23: note: while referencing 'ptrs'
   1527 |                 void *ptrs[0] __aligned(8);
        |                       ^~~~

  [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20230105192646.never.154-kees@kernel.org