datapath: Disable LRO from userspace instead of the kernel.

Whenever a port is added to the datapath, LRO is automatically disabled.
In the future, we may want to enable LRO in some circumstances, so have
userspace disable LRO through the ethtool ioctls.

As part of this change, the MTU and LRO checks are moved to
netdev-vport's send(), which is where they're actually needed.

Feature #6810

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>