bpftool: Stop using deprecated bpf_load_program()

Switch to bpf_prog_load() instead.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211103220845.2676888-7-andrii@kernel.org