misc: lkdtm: bugs: At least try to use popuated variable

The result may not be intereresting, but not using a set variable
is bad form and causes W=1 kernel builds to complain.

Fixes the following W=1 warning(s):

 drivers/misc/lkdtm/bugs.c: In function ‘lkdtm_STACK_GUARD_PAGE_LEADING’:
 drivers/misc/lkdtm/bugs.c:331:25: warning: variable ‘byte’ set but not used [-Wunused-but-set-variable]
 331 | volatile unsigned char byte;
 | ^~~~
 drivers/misc/lkdtm/bugs.c: In function ‘lkdtm_STACK_GUARD_PAGE_TRAILING’:
 drivers/misc/lkdtm/bugs.c:345:25: warning: variable ‘byte’ set but not used [-Wunused-but-set-variable]
 345 | volatile unsigned char byte;
 | ^~~~

Cc: Kees Cook <keescook@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200626130525.389469-5-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm/bugs.c

@@ -333,7 +333,7 @@ void lkdtm_STACK_GUARD_PAGE_LEADING(void)
 
 	byte = *ptr;
 
-	pr_err("FAIL: accessed page before stack!\n");
+	pr_err("FAIL: accessed page before stack! (byte: %x)\n", byte);
 }
 
 /* Test that VMAP_STACK is actually allocating with a trailing guard page */
@@ -347,7 +347,7 @@ void lkdtm_STACK_GUARD_PAGE_TRAILING(void)
 
 	byte = *ptr;
 
-	pr_err("FAIL: accessed page after stack!\n");
+	pr_err("FAIL: accessed page after stack! (byte: %x)\n", byte);
 }
 
 void lkdtm_UNSET_SMEP(void)