raw/ioat: add separate API for fence call

raw/ioat: add separate API for fence call

Rather than having the fence signalled via a flag on a descriptor - which
requires reading the docs to find out whether the flag needs to go on the
last descriptor before, or the first descriptor after the fence - we can
instead add a separate fence API call. This becomes unambiguous to use,
since the fence call explicitly comes between two other enqueue calls. It
also allows more freedom of implementation in the driver code.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Kevin Laatz <kevin.laatz@intel.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>