kcm: use BPF_PROG_RUN

Instead of invoking struct bpf_prog::bpf_func directly, use the
BPF_PROG_RUN macro.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>