[PATCH 22/28] xor: add a better public API

[Dan Williams]

Christoph Hellwig wrote:
> xor_blocks is very annoying to use, because it is limited to 4 + 1
> sources / destinations, has an odd argument order and is completely
> undocumented.
> 
> Lift the code that loops around it from btrfs and async_tx/async_xor into
> common code under the name xor_gen and properly document it.

Apologies for the latency on taking a look at this set from me. This and
the touch to async_xor() look good.

Reviewed-by: Dan Williams <djbw at kernel.org>