net, core: Allow netdev_lower_get_next_private_rcu in bh context

For the XDP bonding slave lookup to work in the NAPI poll context in which
the redudant rcu_read_lock() has been removed we have to follow the same
approach as in 694cea39 ("bpf: Allow RCU-protected lookups to happen
from bh context") and modify the WARN_ON to also check rcu_read_lock_bh_held().

Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210731055738.16820-6-joamaki@gmail.com