x86/cpu_entry_area: Prevent wraparound in setup_cpu_entry_area_ptes() on 32bit

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: kernel test robot <fengguang.wu@intel.com>
Signed-off-by: Thomas "Feels stupid" Gleixner <tglx@linutronix.de>
Tested-by: Borislav Petkov <bp@alien8.de>