bpf, libbpf: Use valid btf in bpf_program__set_attach_target

bpf_program__set_attach_target(prog, fd, ...) will always fail when
fd = 0 (attach to a kernel symbol) because obj->btf_vmlinux is NULL
and there is no way to set it (at the moment btf_vmlinux is meant
to be temporary storage for use in bpf_object__load_xattr()).

Fix this by using libbpf_find_vmlinux_btf_id().

At some point we may want to opportunistically cache btf_vmlinux
so it can be reused with multiple programs.

Signed-off-by: Luigi Rizzo <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Petar Penkov <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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