selftests/bpf: Add reproducer for decl_tag in func_proto return type

It should trigger a WARN_ON_ONCE in btf_type_id_size.

     btf_func_proto_check kernel/bpf/btf.c:4447 [inline]
     btf_check_all_types kernel/bpf/btf.c:4723 [inline]
     btf_parse_type_sec kernel/bpf/btf.c:4752 [inline]
     btf_parse kernel/bpf/btf.c:5026 [inline]
     btf_new_fd+0x1926/0x1e70 kernel/bpf/btf.c:6892
     bpf_btf_load kernel/bpf/syscall.c:4324 [inline]
     __sys_bpf+0xb7d/0x4cf0 kernel/bpf/syscall.c:5010
     __do_sys_bpf kernel/bpf/syscall.c:5069 [inline]
     __se_sys_bpf kernel/bpf/syscall.c:5067 [inline]
     __x64_sys_bpf+0x75/0xb0 kernel/bpf/syscall.c:5067
     do_syscall_x64 arch/x86/entry/common.c:50 [inline]
     do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
     entry_SYSCALL_64_after_hwframe+0x63/0xcd

Cc: Yonghong Song <yhs@fb.com>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221015002444.2680969-1-sdf@google.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>