lib: add test module for CONFIG_DEBUG_VIRTUAL

Add a test module that allows testing that CONFIG_DEBUG_VIRTUAL works
correctly, at least that it can catch invalid calls to virt_to_phys()
against the non-linear kernel virtual address map.

Link: http://lkml.kernel.org/r/20170808164035.26725-1-f.fainelli@gmail.com
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: "Luis R. Rodriguez" <mcgrof@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 7396f50..b19c491 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1930,6 +1930,17 @@
 
 	  If unsure, say N.
 
+config TEST_DEBUG_VIRTUAL
+	tristate "Test CONFIG_DEBUG_VIRTUAL feature"
+	depends on DEBUG_VIRTUAL
+	help
+	  Test the kernel's ability to detect incorrect calls to
+	  virt_to_phys() done against the non-linear part of the
+	  kernel's virtual address map.
+
+	  If unsure, say N.
+
+
 source "samples/Kconfig"
 
 source "lib/Kconfig.kgdb"