RISC-V: KVM: Improve dirty log clearing by skipping zero bits in mask

The existing kvm_riscv_gstage_wp_range() walks the entire [start, end)
range to apply write protection for dirty log clearing, even when the
provided mask has zero bits (i.e., many GFNs do not need protection).
This leads to unnecessary page table walks when the mask is sparse.

Replace the range-based approach with a new function
kvm_riscv_gstage_wp_pt_masked() that iterates only over the set bits in
the mask. For each set bit, it looks up the leaf PTE and applies write
protection. Once a huge page is encountered, the entire huge-page range
is processed in one go, and the corresponding bits in the mask are cleared
using bitmap_clear().

Performance was measured with KVM selftests dirty_log_perf_test on a
Spacemit k3 host with the following configuration:
- vCPUs: 2 (-v 2)
- Memory: 1GB (-b 1G)
- Iterations: 3 (-i 3)
- Write percentage varied via -w parameter to simulate different
  dirty mask densities.

The following data shows the time spent in the clear-dirty-log phase
(i.e., the KVM_CLEAR_DIRTY_LOG ioctl) under each configuration.

+------------------+------------------+------------------+-------------+
| Write Percentage | Original (s)     | Patched (s)      | Improvement |
+------------------+------------------+------------------+-------------+
| 10%              | 0.012905         | 0.009213         | +28.6%      |
| 30%              | 0.014217         | 0.010287         | +27.6%      |
| 50%              | 0.014606         | 0.011772         | +19.4%      |
| 70%              | 0.014735         | 0.013199         | +10.4%      |
| 100%             | 0.014759         | 0.015294         | -3.6%       |
+------------------+------------------+------------------+-------------+

The performance improvement is most significant when the dirty mask is
sparse (low write percentage), which is common in real-world scenarios
with low to moderate memory write intensity. In the worst-case scenario
where the mask is fully set (100% write), the optimization introduces
a slight 3.6% overhead due to the additional bit operations, which is
acceptable given the substantial gains in common cases.

This change significantly reduces the number of page-table walks when
the dirty mask has many zero bits, improving the efficiency of
KVM_CLEAR_DIRTY_LOG and related ioctls.

Signed-off-by: Wang Yechao <wang.yechao255@zte.com.cn>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260717080654.877151-1-wang.yechao255@zte.com.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

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