bpftool: Fix cgroup command to only show cgroup bpf programs

bpftool: Fix cgroup command to only show cgroup bpf programs

The netkit program is not a cgroup bpf program and should not be shown
in the output of the "bpftool cgroup show" command.

However, if the netkit device happens to have ifindex 3,
the "bpftool cgroup show" command will output the netkit
bpf program as well:

> ip -d link show dev nk1
3: nk1@if2: ...
    link/ether ...
    netkit mode ...

> bpftool net show
tc:
nk1(3) netkit/peer tw_ns_nk2phy prog_id 469447

> bpftool cgroup show /sys/fs/cgroup/...
ID       AttachType      AttachFlags     Name
...      ...                             ...
469447   netkit_peer                     tw_ns_nk2phy

The reason is that the target_fd (which is the cgroup_fd here) and
the target_ifindex are in a union in the uapi/linux/bpf.h. The bpftool
iterates all values in "enum bpf_attach_type" which includes
non cgroup attach types like netkit. The cgroup_fd is usually 3 here,
so the bug is triggered when the netkit ifindex just happens
to be 3 as well.

The bpftool's cgroup.c already has a list of cgroup-only attach type
defined in "cgroup_attach_types[]". This patch fixes it by iterating
over "cgroup_attach_types[]" instead of "__MAX_BPF_ATTACH_TYPE".

Cc: Quentin Monnet <qmo@kernel.org>
Reported-by: Takshak Chahande <ctakshak@meta.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/r/20250507203232.1420762-1-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>