selftests/bpf: Remove the only use of deprecated bpf_object__load_xattr()

Switch from bpf_object__load_xattr() to bpf_object__load() and
kernel_log_level in bpf_object_open_opts.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211209193840.1248570-12-andrii@kernel.org