MFC r295273:
In FreeBSD 10 and higher the driver announces SCTP checksum offloading support
also for 82598, which doesn't support it.
The legacy code has a check for it, which was missed when the code for dealing with
CSUM_IP6_* was added. Add the same check for FreeBSD 10 and higher.

Approved by: re (marius)
Differential Revision: D5192