libbpf: Deprecate priv/set_priv storage

Arbitrary storage via bpf_*__set_priv/__priv is being deprecated
without a replacement ([1]). perf uses this capability, but most of
that is going away with the removal of prologue generation ([2]).
perf is already suppressing deprecation warnings, so the remaining
cleanup will happen separately.

  [1]: Closes: https://github.com/libbpf/libbpf/issues/294
  [2]: https://lore.kernel.org/bpf/20220123221932.537060-1-jolsa@kernel.org/



Signed-off-by: Delyan Kratunov <delyank@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220203180032.1921580-1-delyank@fb.com