bpf: udp: Get rid of st_bucket_done

bpf: udp: Get rid of st_bucket_done

Get rid of the st_bucket_done field to simplify UDP iterator state and
logic. Before, st_bucket_done could be false if bpf_iter_udp_batch
returned a partial batch; however, with the last patch ("bpf: udp: 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>