HWPOISON: Add simple debugfs interface to inject hwpoison on arbitary PFNs

Useful for some testing scenarios, although specific testing is often
done better through MADV_POISON

This can be done with the x86 level MCE injector too, but this interface
allows it to do independently from low level x86 changes.

v2: Add module license (Haicheng Li)

Signed-off-by: Andi Kleen <ak@linux.intel.com>

mm/Kconfig

@@ -243,6 +243,10 @@ config MEMORY_FAILURE
 	  even when some of its memory has uncorrected errors. This requires
 	  special hardware support and typically ECC memory.
 
+config HWPOISON_INJECT
+	tristate "Poison pages injector"
+	depends on MEMORY_FAILURE && DEBUG_KERNEL
+
 config NOMMU_INITIAL_TRIM_EXCESS
 	int "Turn on mmap() excess space trimming before booting"
 	depends on !MMU