RISC-V: KVM: Simplify kvm_arch_prepare_memory_region()

In kvm_arch_prepare_memory_region(), if no error occurs, a spin_lock()/
spin_unlock() call can be avoided.

Switch to kvm_riscv_gstage_iounmap() that is the same as the current code,
but with a better semantic.
It also embeds the locking logic. So it is avoided if ret == 0.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>