samples/bpf: Stop using bpf_object__find_program_by_title API.

bpf_object__find_program_by_title is going to be deprecated.
Replace use cases of bpf_object__find_program_by_title in samples/bpf/
with bpf_object__for_each_program.

Signed-off-by: Kui-Feng Lee <kuifeng@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211214035931.1148209-3-kuifeng@fb.com