datapath: Enable offloading on internal devices.

datapath: Enable offloading on internal devices.

Enables checksum offloading, scatter/gather, and TSO on internal
devices.  While these optimizations were not previously enabled on
internal ports we already could receive these types of packets from
Xen guests.  This has the obvious performance benefits when these
packets can be passed directly to hardware.

There is also a more subtle benefit for GRE on Xen.  GRE packets
pass through OVS twice - once before encapsulation and once after
encapsulation, moving through an internal device in the process.
If it is a SG packet (as is common on Xen), a copy was necessary
to linearize for the internal device.  However, Xen uses the
memory allocator to track packets so when the original packet is
freed after the copy netback notifies the guest that the packet
has been sent, despite the fact that it is actually sitting in the
transmit queue.  The guest then sends packets as fast as the CPU
can handle, overflowing the transmit queue.  By enabling SG on
the internal device, we avoid the copy and keep the accounting
correct.

In certain circumstances this patch can decrease performance for
TCP.  TCP has its own mechanism for tracking in-flight packets
and therefore does not benefit from the corrected socket accounting.
However, certain NICs do not like SG when it is not being used for
TSO (these packets can no longer be handled by TSO after GRE
encapsulation).  These NICs presumably enable SG even though they
can't handle it well because TSO requires SG.

Tested controllers (all 1G):
Marvell 88E8053 (large performance hit)
Broadcom BCM5721 (small performance hit)
Intel 82571EB (no change)