bpf: tcp: Get rid of st_bucket_done

Get rid of the st_bucket_done field to simplify TCP iterator state and
logic. Before, st_bucket_done could be false if bpf_iter_tcp_batch
returned a partial batch; however, with the last patch ("bpf: tcp: Make
sure iter->batch always contains a full bucket snapshot"),
st_bucket_done == true is equivalent to iter->cur_sk == iter->end_sk.

Signed-off-by: Jordan Rife <jordan@jrife.io>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>