powerpc/64s/slb: Fix SLB multihit issue during SLB preload

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CVE-2025-71078

A Segment Lookaside Buffer SLB multi-hit error vulnerability was found in the Linux kernel's PowerPC 64-bit hash MMU implementation. When a process migrates between CPUs without triggering switchmmucontext because prev and next mmstruct are the same, the hardware SLB and software preload cache ca...