Something is broken if we take this path. This routine could stand to
have a WARN_ONCE(), because if it is ever called there is something is
wrong with the raid6 offload driver. The intent is that the
asynchronous recovery routines will perform an early check to see if
the recovery can be offloaded (with a series of calls to
async_xor/async_pq). If not we fall back to the optimized synchronous
recovery routines (raid6_2data_recov, raid6_datap_recov). The only
time this path will be taken is if we have decided to perform
asynchronous recovery but at a later call to async_pq the offload
driver reports it has run out of descriptors.