CVE-2021-47018

In the Linux kernel, the following vulnerability has been resolved:
powerpc/64: Fix the definition of the fixmap area At the time being, the
fixmap area is defined at the top of the address space or just below KASAN.
This definition is not valid for PPC64. For PPC64, use the top of the I/O
space. Because of circular dependencies, it is not possible to include
asm/fixmap.h in asm/book3s/64/pgtable.h , so define a fixed size AREA at
the top of the I/O space for fixmap and ensure during build that the size
is big enough.