selftests/bpf: Rename tcp-cc private struct in bpf_cubic and bpf_dctcp

selftests/bpf: Rename tcp-cc private struct in bpf_cubic and bpf_dctcp

The "struct bictcp" and "struct dctcp" are private to the bpf prog
and they are stored in the private buffer in inet_csk(sk)->icsk_ca_priv.
Hence, there is no bpf CO-RE required.

The same struct name exists in the vmlinux.h. To reuse vmlinux.h,
they need to be renamed such that the bpf prog logic will be
immuned from the kernel tcp-cc changes.

This patch adds a "bpf_" prefix to them.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20240509175026.3423614-6-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>