bpf: Factor callchain_finalize function from __bpf_get_stack

The new callchain_finalize function calls the build-id retrieval
(if needed) and zeroes the buffer. This makes things easier for
preemption fix in following change.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260803210149.296496-8-jolsa@kernel.org