x86 KASLR base exposed due to lack of entry trampoline (EntryBleed, CVE-2022-4543)

[kees]

https://www.willsroot.io/2022/12/entrybleed.html

From v4.20 on, after KPTI was implemented, which had a fixed-location syscall entry trampoline, the trampoline was removed. This exposes the actual kernel mapping address via prefetch, etc. We need to restore this trampoline.