mm/ptdump: take the memory hotplug lock inside ptdump_walk_pgd()

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CVE-2025-38681

CVE-2025-38681 affects the Linux kernel mm/ptdump code. The issue arises when memory hotplug modifications race with ptdump_walk_pgd() and intermediate page-table frees, causing the ptdump code to dereference freed memory and potentially crash or corrupt data. The fix moves the memory hotplug loc...