netdevsim: make bpf_offload_dev_create() per-sdev instead of first ns

offload dev is stored in sdev struct. However, first netdevsim instance
is used as a priv. Change this to be sdev to as it is shared among
multiple netdevsim instances.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>