macvlan: allow setting LRO independently of lower device

Since commit fbe168ba ("net: generic dev_disable_lro() stacked
device handling"), dev_disable_lro() zeroes NETIF_F_LRO feature flag
first for a macvlan device and then for its lower device. As an attempt
to set NETIF_F_LRO to zero is ignored, dev_disable_lro() issues a
warning and taints kernel.

Allowing NETIF_F_LRO to be set independently of the lower device
consists of three parts:

  - add the flag to hw_features to allow toggling it
  - allow setting it to 0 even if lower device has the flag set
  - add the flag to MACVLAN_FEATURES to restore copying from lower
    device on macvlan creation

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>