selftests/bpf: Check -EBUSY for the recurred bpf_setsockopt(TCP_CONGESTION)

This patch changes the bpf_dctcp test to ensure the recurred
bpf_setsockopt(TCP_CONGESTION) returns -EBUSY.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20220929070407.965581-6-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>