KVM: riscv: Implement KVM_PRE_FAULT_MEMORY

The generic KVM_PRE_FAULT_MEMORY ioctl lets userspace populate KVM page
tables before running a vCPU over a GPA range. x86 already supports the
ioctl, but RISC-V does not expose the capability and has no arch hook.

Add the RISC-V arch hook and reuse the existing G-stage fault mapping
path with a read access. Report progress using the G-stage mapping
returned by the map path, so the ioctl can advance by the actual leaf
size that covers the requested GPA. Retry until a mapping is installed
or a signal, VM-dead request, or real error is observed.

Signed-off-by: Jinyu Tang <jinyu.tang@linux.dev>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260814114507.583424-2-jinyu.tang@linux.dev
Signed-off-by: Anup Patel <anup@brainfault.org>

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