IPoIB: Set netdev offload features properly for child (VLAN) interfaces

Child devices were created without any offload features set, fix this by
moving the code that computes the features into generic function which is
now called through non-child and child device creation.

Signed-off-by: Or Gerlitz <ogerlitz@voltaire.com>
-- v1 has a bug where the 'result' flag in ipoib_vlan_add may be used uninitialized
Signed-off-by: Roland Dreier <rolandd@cisco.com>