KVM: riscv: Use an rwlock for mmu_lock

RISC-V KVM currently uses a spinlock for mmu_lock. That serializes all
G-stage MMU operations, including permission-only updates that do not
allocate or free page-table pages.

Use KVM's rwlock form of mmu_lock, as x86 and arm64 already do. Keep the
existing map, unmap and teardown paths on the write side. This prepares
RISC-V for read-side handling of G-stage permission updates.

Signed-off-by: Jinyu Tang <tjytimi@163.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260517153427.94889-3-tjytimi@163.com
Signed-off-by: Anup Patel <anup@brainfault.org>

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