bpf: task_group_seq_get_next: kill next_task

It only adds the unnecessary confusion and compicates the "retry" code.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230905154654.GA24945@redhat.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>