md/raid6: asynchronous raid6 operations

[ Based on an original patch by Yuri Tikhonov ]

The raid_run_ops routine uses the asynchronous offload api and
the stripe_operations member of a stripe_head to carry out xor+pq+copy
operations asynchronously, outside the lock.

The operations performed by RAID-6 are the same as in the RAID-5 case
except for no support of STRIPE_OP_PREXOR operations. All the others
are supported:
STRIPE_OP_BIOFILL
 - copy data into request buffers to satisfy a read request
STRIPE_OP_COMPUTE_BLK
 - generate missing blocks (1 or 2) in the cache from the other blocks
STRIPE_OP_BIODRAIN
 - copy data out of request buffers to satisfy a write request
STRIPE_OP_RECONSTRUCT
 - recalculate parity for new data that has entered the cache
STRIPE_OP_CHECK
 - verify that the parity is correct

The flow is the same as in the RAID-5 case, and reuses some routines, namely:
1/ ops_complete_postxor (renamed to ops_complete_reconstruct)
2/ ops_complete_compute (updated to set up to 2 targets uptodate)
3/ ops_run_check (renamed to ops_run_check_p for xor parity checks)

[neilb@suse.de: fixes to get it to pass mdadm regression suite]
Reviewed-by: Andre Noll <maan@systemlinux.org>
Signed-off-by: Yuri Tikhonov <yur@emcraft.com>
Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/md/Kconfig

@@ -124,6 +124,8 @@ config MD_RAID456
 	select MD_RAID6_PQ
 	select ASYNC_MEMCPY
 	select ASYNC_XOR
+	select ASYNC_PQ
+	select ASYNC_RAID6_RECOV
 	---help---
 	  A RAID-5 set of N drives with a capacity of C MB per drive provides
 	  the capacity of C * (N - 1) MB, and protects against a failure

drivers/md/raid5.h

@@ -176,7 +176,9 @@
  */
 enum check_states {
 	check_state_idle = 0,
-	check_state_run, /* parity check */
+	check_state_run, /* xor parity check */
+	check_state_run_q, /* q-parity check */
+	check_state_run_pq, /* pq dual parity check */
 	check_state_check_result,
 	check_state_compute_run, /* parity repair */
 	check_state_compute_result,
@@ -216,7 +218,7 @@ struct stripe_head {
 	 * @target - STRIPE_OP_COMPUTE_BLK target
 	 */
 	struct stripe_operations {
-		int		     target;
+		int 		     target, target2;
 		enum sum_check_flags zero_sum_result;
 	} ops;
 	struct r5dev {
@@ -299,7 +301,7 @@ struct r6_state {
 #define STRIPE_OP_COMPUTE_BLK	1
 #define STRIPE_OP_PREXOR	2
 #define STRIPE_OP_BIODRAIN	3
-#define STRIPE_OP_POSTXOR	4
+#define STRIPE_OP_RECONSTRUCT	4
 #define STRIPE_OP_CHECK	5
 
 /*