bond: set offload capabilities flags

bond: set offload capabilities flags

Before the fix, bond device's offload capabilities are unset. This fix
takes the minimum common set of slave devices' capabilities as bond
device's capabilities. For simplicity, we ensure all slave devices
to have a capability before bond device can claim this capability,
even if some slave devices are unused (i.e. linked down, standby).

Signed-off-by: Jia Yu <jyu@vmware.com>
Signed-off-by: Declan Doherty <declan.doherty@intel.com>