libbpf: Split bpf_core_apply_relo() into bpf_program independent helper.

bpf_core_apply_relo() doesn't need to know bpf_program internals
and hashmap details.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210721000822.40958-3-alexei.starovoitov@gmail.com