KVM: riscv: Fix Spectre-v1 in vector register access

KVM: riscv: Fix Spectre-v1 in vector register access

User-controlled register indices from the ONE_REG ioctl are used to
index into the vector register buffer (v0..v31). Sanitize the calculated
offset with array_index_nospec() to prevent speculative out-of-bounds
access.

Signed-off-by: Zongmin Zhou <zhouzongmin@kylinos.cn>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260715030818.75657-1-min_halo@163.com
Signed-off-by: Anup Patel <anup@brainfault.org>