misc lib/raid/ improvements v2

Hi all,

this series improves the benchmark-based algorithm selection and
adds kunit benchmarks for both the XOR and raid6 libraries, and tidies
up a few very minor other bits.

Note that Sashiko still complains about long preempt_disable regions.
While that is right in general, it is very much intentional for these
kunit boot / module load time benchmarks.

Changes since v1:
 - fix the deferred implementation backing
 - remove pointless variable shadowing
 - fix up a commit message
 - don't add a duplicate RAID6_PQ symbol
 - fix the calculated RAID6 "XOR" bandwidth
 - floor all timing delta to 1 to avoid entire hypothetical
   divide by zero errors

diffstat:
 Kconfig                   |   12 +++++
 raid6/Makefile            |    2 
 raid6/algos.c             |  102 +++++++++++++++++++++++++---------------------
 raid6/tests/raid6_kunit.c |   66 +++++++++++++++++++++++++++++
 xor/Makefile              |    2 
 xor/tests/Makefile        |    2 
 xor/tests/xor_kunit.c     |   64 ++++++++++++++++++++++++++++
 xor/xor-core.c            |   55 +++++++++++-------------
 8 files changed, 228 insertions(+), 77 deletions(-)