KVM: RISC-V: reset smstateen CSRs

Not resetting smstateen is a potential security hole, because VU might
be able to access state that VS does not properly context-switch.

Fixes: 81f0f314 ("RISCV: KVM: Add sstateen0 context save/restore")
Signed-off-by: Radim Krčmář <rkrcmar@ventanamicro.com>
Link: https://lore.kernel.org/r/20250403112522.1566629-8-rkrcmar@ventanamicro.com


Signed-off-by: Anup Patel <anup@brainfault.org>