Split CFI jump tables between core kernel and entry code so KPTI doesn't have to map the entire jump table

[kees]

https://outflux.net/blog/archives/2019/11/20/experimenting-with-clang-cfi-in-upstream-linux/

There seems to be some generated code that is being run at syscall entry that is not mapped. I suspect there is something still doing CFI checks on entry (and their resulting jump tables are missing), but I haven't found them yet. For now, disable KPTI under CFI (at run time, so test kernels can still boot with "pti" to enable it again).

[kees]

Looks like current best solution is to just always map the jump tables.

[kees]

No longer an issue; the entry code is using the opaque types to avoid CFI instrumentation.