RISC-V: KVM: Batch stage-2 TLB flushes

KVM RISC-V triggers a TLB flush for every single stage-2 PTE
modification (unmap or write-protect) now. Although KVM coalesces the
hardware IPIs, the software overhead of executing the flush work
for every page is large, especially during dirty page tracking.

Following the approach used in x86 and arm64, this patch optimizes
the MMU logic by making the PTE manipulation functions return a boolean
indicating if a leaf PTE was actually changed. The outer MMU functions
bubble up this flag to batch the remote TLB flushes.

Consequently, the flush operation is executed only once per batch.
Moving it outside of the `mmu_lock` also reduces lock contention.

Tested with tools/testing/selftests/kvm on a 4-vCPU guest (Host
environment: QEMU 10.2.1 RISC-V)
1. demand_paging_test (1GB memory)
  time ./demand_paging_test -b 1G -v 4
- Total execution time reduced from ~2m39s to ~2m31s
2. dirty_log_perf_test (1GB memory)
  ./dirty_log_perf_test -b 1G -v 4
- "Clear dirty log time" per iteration dropped significantly from
   ~3.40s to ~0.18s

Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Jinyu Tang <tjytimi@163.com>
Link: https://lore.kernel.org/r/20260412023822.83341-1-tjytimi@163.com
Signed-off-by: Anup Patel <anup@brainfault.org>

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