arm64: Drop unneeded stackleak_check_alloca()

Drop stackleak_check_alloca() for arm64 since the STACKLEAK gcc plugin now
doesn't track stack depth overflow caused by alloca().
Signed-off-by: Alexander Popov <alex.popov@linux.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

arch/arm64/kernel/process.c

@@ -493,25 +493,3 @@ void arch_setup_new_exec(void)
 {
 	current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;
 }
-
-#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
-void __used stackleak_check_alloca(unsigned long size)
-{
-	unsigned long stack_left;
-	unsigned long current_sp = current_stack_pointer;
-	struct stack_info info;
-
-	BUG_ON(!on_accessible_stack(current, current_sp, &info));
-
-	stack_left = current_sp - info.low;
-
-	/*
-	 * There's a good chance we're almost out of stack space if this
-	 * is true. Using panic() over BUG() is more likely to give
-	 * reliable debugging output.
-	 */
-	if (size >= stack_left)
-		panic("alloca() over the kernel stack boundary\n");
-}
-EXPORT_SYMBOL(stackleak_check_alloca);
-#endif