arm64: unexport walk_stackframe

The walk_stackframe functions is architecture-specific, with a varying
prototype, and common code should not use it directly. None of its
current users can be built as modules. With THREAD_INFO_IN_TASK, users
will also need to hold a stack reference before calling it.

There's no reason for it to be exported, and it's very easy to misuse,
so unexport it for now.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/stacktrace.c

@@ -129,7 +129,6 @@ void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
 			break;
 	}
 }
-EXPORT_SYMBOL(walk_stackframe);
 
 #ifdef CONFIG_STACKTRACE
 struct stack_trace_data {