RISC-V: KVM: Cache gstage pgd_levels in struct kvm_gstage

RISC-V: KVM: Cache gstage pgd_levels in struct kvm_gstage

Gstage page-table helpers frequently chase gstage->kvm->arch to
fetch pgd_levels. This adds noise and repeats the same dereference
chain in hot paths.

Add pgd_levels to struct kvm_gstage and initialize it from kvm->arch
when setting up a gstage instance. Introduce kvm_riscv_gstage_init()
to centralize initialization and switch gstage code to use
gstage->pgd_levels.

Suggested-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Fangyu Yu <fangyu.yu@linux.alibaba.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>
Link: https://lore.kernel.org/r/20260403153019.9916-3-fangyu.yu@linux.alibaba.com
Signed-off-by: Anup Patel <anup@brainfault.org>