net offloading: Convert skb_need_linearize() to use precomputed features.

This switches skb_need_linearize() to use the features that have
been centrally computed.  In doing so, this fixes a problem where
scatter/gather should not be used because the card does not support
checksum offloading on that type of packet.  On device registration
we only check that some form of checksum offloading is available if
scatter/gatther is enabled but we must also check at transmission
time.  Examples of this include IPv6 or vlan packets on a NIC that
only supports IPv4 offloading.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>