[$] Meltdown/Spectre mitigation for 4.15 and beyond

While some aspects of the kernel's defenses against the Meltdown andSpectre vulnerabilities were more-or-less in place when the problems weredisclosed on January 3, others were less fully formed. Additionally,many of the mitigations (especially for the two Spectre variants) had notbeen seen in public prior to the disclosure, meaning that there was a lotof scope for discussion once they came out. Many of those discussions areslowing down, and the kernel's initial response has mostly come intofocus. The 4.15 kernel will include a broad set of mitigations, while someothers will have to wait for later; read onfor details on where things stand.