bpf: Factor stackid_new_bucket from __bpf_get_stackid

The new stackid_new_bucket allocates the new bucket and initializes it
with the trace data.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260803210149.296496-4-jolsa@kernel.org