MFC r302499: Improve checksum "offload" support.

For compatibility reasons make driver not report any checksum offload by
default, since there is indeed none. But if administrator knows that
interface is used only for local traffic, he can enable fake checksum
offload manually on both sides to save some CPU cycles, since the data
are already protected by CRC32 of PCIe link.

Sponsored by: iXsystems, Inc.