etmem: fix potential UAF when walk ept page table

euleros inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7RO5Q
CVE: NA
Reference: https://gitee.com/openeuler/kernel/commit/0bb20a8fa7e49d95a179337f54e3d23be1ea351f



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Kvm shadow page may be freed when etmem_scan is walking ept page table.
Hold mmu_lock when walking ept page table to avoid UAF.
To avoid holding mmu_lock for too long time, walk step module parameter
is added to control lock holding time.

Signed-off-by: Kemeng Shi <shikemeng@huawei.com>
Reviewed-by: louhongxiang <louhongxiang@huawei.com>
Reviewed-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>