datapath: Assert IFF_TX_SKB_SHARING on internal devices.

Linux 3.1 adds a flag to check whether it's OK for shared skbs to
be transmitted on devices.  This generally isn't a problem for
hardware devices but software devices such as OVS that hold state
in the skb need to clear the flag, which is enabled by default.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>