[$] Page allocation for address-space isolation

Address-space isolation may well be, as Brendan Jackman said at thebeginning of his memory-management-track session at the 2025 Linux Storage,Filesystem, Memory-Management, and BPF Summit, "some securitybullshit". But it also holds the potential to protect the kernel froma wide range of vulnerabilities, both known and unknown, while reducing theimpact of existing mitigations. Implementing address-space isolation withreasonable performance, though, is going to require some significantchanges. Jackman was there to get feedback from the memory-managementcommunity on how those changes should be implemented.