bpf: Fix missing prog untrack in release_maps

Commit da765a2f ("bpf: Add poke dependency tracking for prog array
maps") wrongly assumed that in case of prog load errors, we're cleaning
up all program tracking via bpf_free_used_maps().

However, it can happen that we're still at the point where we didn't copy
map pointers into the prog's aux section such that env->prog->aux->used_maps
is still zero, running into a UAF. In such case, the verifier has similar
release_maps() helper that drops references to used maps from its env.

Consolidate the release code into __bpf_free_used_maps() and call it from
all sides to fix it.

Fixes: da765a2f

 ("bpf: Add poke dependency tracking for prog array maps")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/1c2909484ca524ae9f55109b06f22b6213e76376.1576514756.git.daniel@iogearbox.net