pq - generate the p+q (raid6 syndrome) from a series of source buffers
pq_val - validate that a p and or q buffer are in sync with a given series of
sources
+datap - (raid6_datap_recov) recover a raid6 data block and the p block
+ from the given sources
+2data - (raid6_2data_recov) recover 2 raid6 data blocks from the given
+ sources
3.3 Descriptor management:
The return value is non-NULL and points to a 'descriptor' when the operation