lib/raid6: Include <asm/ppc-opcode.h> for VPERMXOR

On Ubuntu 21.10 (ppc64le) building raid6test with gcc (Ubuntu
11.2.0-7ubuntu2) 11.2.0 fails with the error below.

gcc -I.. -I ../../../include -g -O2 \
-I../../../arch/powerpc/include -DCONFIG_ALTIVEC \
-c -o vpermxor1.o vpermxor1.c
vpermxor1.c: In function ‘raid6_vpermxor1_gen_syndrome_real’:
vpermxor1.c:64:29: error: expected string literal before ‘VPERMXOR’
64 | asm(VPERMXOR(%0,%1,%2,%3):"=v"(wq0):"v"(gf_high), "v"(gf_low), "v"(wq0));
| ^~~~~~~~
make: *** [Makefile:58: vpermxor1.o] Error 1

So, include the header asm/ppc-opcode.h defining this macro also when
not building the Linux kernel but only this too.

Cc: Matt Brown <matthew.brown.dev@gmail.com>
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Song Liu <song@kernel.org>

lib/raid6/altivec: Add vpermxor implementation for raid6 Q syndrome

This patch uses the vpermxor instruction to optimise the raid6 Q
syndrome. This instruction was made available with POWER8, ISA version
2.07. It allows for both vperm and vxor instructions to be done in a
single instruction. This has been tested for correctness on a ppc64le
vm with a basic RAID6 setup containing 5 drives.

The performance benchmarks are from the raid6test in the
/lib/raid6/test directory. These results are from an IBM Firestone
machine with ppc64le architecture. The benchmark results show a 35%
speed increase over the best existing algorithm for powerpc (altivec).
The raid6test has also been run on a big-endian ppc64 vm to ensure it
also works for big-endian architectures.

Performance benchmarks:
raid6: altivecx4 gen() 18773 MB/s
raid6: altivecx8 gen() 19438 MB/s

raid6: vpermxor4 gen() 25112 MB/s
raid6: vpermxor8 gen() 26279 MB/s

Signed-off-by: Matt Brown <matthew.brown.dev@gmail.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
[mpe: Add VPERMXOR macro so we can build with old binutils]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>