libbpf: Clean gen_loader's attach kind.

libbpf: Clean gen_loader's attach kind.

[ Upstream commit 19250f5fc0c283892a61f3abf9d65e6325f63897 ]

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: 67234743736a ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>