mana: add ioctl to support toggling offloading features

With this support, users can enable or disable offloading features
such as txcsum, rxcsum, tso and software lro through ifconfig.

To enable or disable tx features, do it on mana interface first,
then hn/netvsc to sync it up with mana. For example:

ifconfig mana0 -txcsum
ifconfig hn0 -tscsum

To enable or disable rx features, just applying on mana interface
would be sufficient.

Disabling txcsum imples disabling tso. Enabling tso when txcsum
is disabled will result in an error message in dmesg requesting
to enable txcsum first.

Above applies to ipv6 offloading features as well.

Tested by: whu
MFC after: 3 days
Sponsored by: Microsoft