bpf: Use stack id functions instead of __bpf_get_stackid

Replacing __bpf_get_stackid calls with sequence of following functions:

  stackid_fastpath
  stackid_new_bucket
  stackid_install

This makes code more structured and allows us to easily disable
preemption only in bpf_get_stackid 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/20260803210149.296496-5-jolsa@kernel.org