selftests/bpf: rename bpf_iter_task.c to bpf_iter_tasks.c

The newly-added struct bpf_iter_task has a name collision with a selftest
for the seq_file task iter's bpf skel, so the selftests/bpf/progs file is
renamed in order to avoid the collision.

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231018061746.111364-8-zhouchuyi@bytedance.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>