When ethtool is calling into devlink compat code make sure we have
a reference on the netdevice on which the operation was invoked.

v3: move the hold/lock logic into devlink_compat_* functions (Florian)
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Acked-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>