bpf: task_group_seq_get_next: fix the skip_if_dup_files check

Unless I am notally confused it is wrong. We are going to return or
skip next_task so we need to check next_task-files, not task->files.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230905154651.GA24940@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>