datapath: Attempt to checksum packets sent to controller on non-Xen kernel.

datapath: Attempt to checksum packets sent to controller on non-Xen kernel.

Commit 96660ad113 "datapath: Fix up checksum on Xen before forwarding to
controller" made sure that packets sent to the controller were properly
checksummed on Xen, where it happens pretty commonly that they are not.
It seems possible that on non-Xen machines this could also happen, even
though it does not seem to be common, so this commit tries to fix up
packets in that case also.

This commit adds a WARN_ON_ONCE() call to make it clear that it has
triggered.  If we ever see this warning, then we should figure out what
triggered it and make sure that this code actually works properly.