lib/bpf: fix verbose flag when using libbpf

lib/bpf: fix verbose flag when using libbpf

Since commit 6d61a2b55799 ("lib: add libbpf support"), passing the
verbose flag to tc filter doesn't dump the verifier logs anymore in case
of successful loading.

This commit fixes it by setting the log_level attribute before loading.
To that end, we need to call bpf_object__load_xattr directly instead of
relying on bpf_object__load.

Fixes: 6d61a2b55799 ("lib: add libbpf support")
Signed-off-by: Paul Chaignon <paul@isovalent.com>
Acked-by: Hangbin Liu <haliu@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>