The loop which populates the CPU entry area PMDs can wrap around on 32bit
machines when the number of CPUs is small.

It worked wonderful for NR_CPUS=64 for whatever reason and the moron who
wrote that code did not bother to test it with !SMP.

Check for the wraparound to fix it.

Fixes: 92a0f81d ("x86/cpu_entry_area: Move it out of the fixmap")
Reported-by: Nkernel test robot <fengguang.wu@intel.com>
Signed-off-by: NThomas "Feels stupid" Gleixner <tglx@linutronix.de>
Tested-by: NBorislav Petkov <bp@alien8.de>