raid6: riscv: Fix NULL pointer dereference caused by a missing clobber

raid6: riscv: Fix NULL pointer dereference caused by a missing clobber

When running the raid6 user-space test program on RISC-V QEMU, there's a
segmentation fault which seems caused by accessing a NULL pointer,
which is the pointer variable p/q in raid6_rvv*_gen/xor_syndrome_real(),
p/q should have been equal to dptr[x], but when I use GDB command to
see its value, which was 0x10 like below:

"
Program received signal SIGSEGV, Segmentation fault.
0x0000000000011062 in raid6_rvv2_xor_syndrome_real (disks=<optimized out>, start=0, stop=<optimized out>, bytes=4096, ptrs=<optimized out>) at rvv.c:386
(gdb) p p
$1 = (u8 *) 0x10 <error: Cannot access memory at address 0x10>
"

The issue was found to be related with:
1) Compile optimization
   There's no segmentation fault if compiling the raid6test program with
   the optimization flag -O0.
2) The RISC-V vector command vsetvli
   If not used t0 as the first parameter in vsetvli, there's no
   segmentation fault either.

This patch selects the 2nd solution to fix the issue.

[Palmer: The actual issue here is a missing clobber in the vsetvli code.
It's a little tricky: we've already probed for VLENB so we don't need to
look at the output register, we just need to have an X register in the
instruction as that's the form required to actually set VL.  Thus we
clobber a register, and without describing that we end up breaking
compilers.]

Fixes: 6093faaf9593 ("raid6: Add RISC-V SIMD syndrome and recovery calculations")
Signed-off-by: Chunyan Zhang <zhangchunyan@iscas.ac.cn>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250610101234.1100660-3-zhangchunyan@iscas.ac.cn
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>