bpf: Avoid -Wflex-array-members-not-at-end warnings

bpf: Avoid -Wflex-array-members-not-at-end warnings

Apparently, struct bpf_empty_prog_array exists entirely to populate a
single element of "items" in a global variable. "null_prog" is only
used during the initializer.

None of this is needed; globals will be correctly sized with an array
initializer of a flexible-array member.

So, remove struct bpf_empty_prog_array and adjust the rest of the code,
accordingly.

With these changes, fix the following warnings:

./include/linux/bpf.h:2369:31: warning: structure containing a flexible
array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/acr7Whmn0br3xeBP@kspp
Signed-off-by: Alexei Starovoitov <ast@kernel.org>