perf: Stop using bpf_object__open_buffer() API

bpf_object__open_buffer() API is deprecated, use the unified opts
bpf_object__open_mem() API in perf instead. This requires at least
libbpf 0.0.6.

Signed-off-by: Christy Lee <christylee@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220125005923.418339-3-christylee@fb.com