RISC-V: KVM: Introduce struct kvm_gstage_mapping

RISC-V: KVM: Introduce struct kvm_gstage_mapping

Introduce struct kvm_gstage_mapping which represents a g-stage
mapping at a particular g-stage page table level. Also, update
the kvm_riscv_gstage_map() to return the g-stage mapping upon
success.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Tested-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Nutty Liu <liujingqi@lanxincomputing.com>
Link: https://lore.kernel.org/r/20250618113532.471448-10-apatel@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>