This is the companion change of D17910:
Once unsupported / unneeded hardware flags are disabled from netmap,
it is needed to propagate them to kernel via ifp->if_hwassist. Otherwise,
packet might be sent without checksum as the kernel was expecting
the NIC to do it.
Submitted by: Marc de la Gueronniere <mdelagueronniere@verisign.com>
MFC after: 1 week
Sponsored by: Verisign, Inc.