bpf: Avoid changing callchain in bpf_get_stackid_pe

There's no need to modify the trace object bpf_get_stackid_pe, we just
need to pass the needed callchain length in separate argument.

This way we can have callchain pointers const and remove the trace->nr
modification and restoration.

Assisted-by: Codex:GPT-5.5

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260803210149.296496-13-jolsa@kernel.org