async_tx: add support for asynchronous RAID6 recovery operations

 async_raid6_2data_recov() recovers two data disk failures

 async_raid6_datap_recov() recovers a data disk and the P disk

These routines are a port of the synchronous versions found in
drivers/md/raid6recov.c.  The primary difference is breaking out the xor
operations into separate calls to async_xor.  Two helper routines are
introduced to perform scalar multiplication where needed.
async_sum_product() multiplies two sources by scalar coefficients and
then sums (xor) the result.  async_mult() simply multiplies a single
source by a scalar.

This implemention also includes, in contrast to the original
synchronous-only code, special case handling for the 4-disk and 5-disk
array cases.  In these situations the default N-disk algorithm will
present 0-source or 1-source operations to dma devices.  To cover for
dma devices where the minimum source count is 2 we implement 4-disk and
5-disk handling in the recovery code.

[ Impact: asynchronous raid6 recovery routines for 2data and datap cases ]

Cc: Yuri Tikhonov <[email protected]>
Cc: Ilya Yanok <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: David Woodhouse <[email protected]>
Reviewed-by: Andre Noll <[email protected]>
Acked-by: Maciej Sosnowski <[email protected]>
Signed-off-by: Dan Williams <[email protected]>

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