ioat: close potential BUG_ON race in the descriptor cleanup path

Since ioat_cleanup_preamble() and the update of the last completed
descriptor are not synchronized there is a chance that two cleanup threads
can see descriptors to clean.  If the first cleans up all pending
descriptors then the second will trigger the BUG_ON.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>