bpf: Factor stackid_init function from __bpf_get_stackid

The new stackid_init function stores all the necessary bits for stackid
trace and it will be used by other functions in following changes.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260803210149.296496-2-jolsa@kernel.org