libbpf: Clean gen_loader's attach kind.

The gen_loader has to clear attach_kind otherwise the programs
without attach_btf_id will fail load if they follow programs
with attach_btf_id.

Fixes: 67234743 ("libbpf: Generate loader program out of BPF ELF file.")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211201181040.23337-12-alexei.starovoitov@gmail.com