riscv: Cleanup stacktrace

1. add asm/stacktrace.h for walk_stackframe and struct stackframe
2. remove unnecessary blank lines in stacktrace.c
3. fix warning "no previous prototype for ‘fill_callchain’"

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/stacktrace.h

@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _ASM_RISCV_STACKTRACE_H
+#define _ASM_RISCV_STACKTRACE_H
+
+#include <linux/sched.h>
+#include <asm/ptrace.h>
+
+struct stackframe {
+	unsigned long fp;
+	unsigned long ra;
+};
+
+extern void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
+				    bool (*fn)(unsigned long, void *), void *arg);
+
+#endif /* _ASM_RISCV_STACKTRACE_H */