bpf: Fix flexible_array.cocci warnings

Zero-length and one-element arrays are deprecated, see:
Documentation/process/deprecated.rst

Flexible-array members should be used instead.

Generated by: scripts/coccinelle/misc/flexible_array.cocci

Fixes: c1ff181ffa ("selftests/bpf: Extend kfunc selftests")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: kernel test robot <lkp@intel.com>
Signed-off-by: Julia Lawall <julia.lawall@inria.fr>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/alpine.DEB.2.22.394.2201221206320.12220@hadrien

net/bpf/test_run.c

@@ -324,7 +324,7 @@ struct prog_test_fail2 {
 struct prog_test_fail3 {
 	int len;
 	char arr1[2];
-	char arr2[0];
+	char arr2[];
 };
 
 noinline void bpf_kfunc_call_test_pass_ctx(struct __sk_buff *skb)