gcc-plugins/stackleak: Don't instrument itself

There is no need to try instrumenting functions in kernel/stackleak.c.
Otherwise that can cause issues if the cleanup pass of stackleak gcc plugin
is disabled.
Signed-off-by: Alexander Popov <alex.popov@linux.com>
Link: https://lore.kernel.org/r/20200624123330.83226-2-alex.popov@linux.comSigned-off-by: Kees Cook <keescook@chromium.org>

kernel/Makefile

@@ -125,6 +125,7 @@ obj-$(CONFIG_WATCH_QUEUE) += watch_queue.o
 
 obj-$(CONFIG_SYSCTL_KUNIT_TEST) += sysctl-test.o
 
+CFLAGS_stackleak.o += $(DISABLE_STACKLEAK_PLUGIN)
 obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += stackleak.o
 KASAN_SANITIZE_stackleak.o := n
 KCSAN_SANITIZE_stackleak.o := n