lkdtm: Test VMAP_STACK allocates leading/trailing guard pages

Two new tests STACK_GUARD_PAGE_LEADING and STACK_GUARD_PAGE_TRAILING
attempt to read the byte before and after, respectively, of the current
stack frame, which should fault.

Signed-off-by: Kees Cook <keescook@chromium.org>

drivers/misc/lkdtm.h

@@ -22,6 +22,8 @@ void lkdtm_HUNG_TASK(void);
 void lkdtm_CORRUPT_LIST_ADD(void);
 void lkdtm_CORRUPT_LIST_DEL(void);
 void lkdtm_CORRUPT_USER_DS(void);
+void lkdtm_STACK_GUARD_PAGE_LEADING(void);
+void lkdtm_STACK_GUARD_PAGE_TRAILING(void);
 
 /* lkdtm_heap.c */
 void lkdtm_OVERWRITE_ALLOCATION(void);