samples/bpf: Relicense bpf_insn.h as GPL-2.0-only OR BSD-2-Clause

libbpf and bpftool have been dual-licensed to facilitate inclusion in software
that is not compatible with GPL2-only (ie: Apache2), but the samples are still
GPL2-only.

Given these files are samples, they get naturally copied around. For example,
it is the case for samples/bpf/bpf_insn.h which was copied into the systemd
tree: https://github.com/systemd/systemd/blob/main/src/shared/linux/bpf_insn.h

Some more context on systemd's needs specifically:

  Most of systemd is (L)GPL2-or-later, which means there is no perceived
  incompatibility with Apache2 software and can thus be linked with
  OpenSSL 3.0. But given this GPL2-only header is included this is currently
  not possible. Dual-licensing this header solves this problem for us as we
  are scoping a move to OpenSSL 3.0, see:

  https://lists.freedesktop.org/archives/systemd-devel/2021-September/046882.html

Dual-license this header as GPL-2.0-only OR BSD-2-Clause to follow the same
licensing used by libbpf and bpftool:

  1bc38b8f ("libbpf: relicense libbpf as LGPL-2.1 OR BSD-2-Clause")
  907b2236 ("tools: bpftool: dual license all files")
Signed-off-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Daniel Mack <daniel@zonque.org>
Acked-by: Josef Bacik <josef@toxicpanda.com>
Acked-by: Joe Stringer <joe@ovn.org>
Acked-by: Chenbo Feng <fengc@google.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Brendan Jackman <jackmanb@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210923000540.47344-1-luca.boccassi@gmail.com

samples/bpf/bpf_insn.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) */
 /* eBPF instruction mini library */
 #ifndef __BPF_INSN_H
 #define __BPF_INSN_H