MFC r365871. This needs a couple other revisions which aren't in
stable/12 yet to actually work but is being committed out of order to
ease other cxgbe MFCs.
cxgbe(4): add support for stateless offloads for VXLAN traffic.
Hardware assistance includes checksumming (tx and rx), TSO, and RSS on
the inner traffic in a VXLAN tunnel.
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