riscv/stacktrace: Fix stack output without ra on the stack top

When a function doesn't have a callee, then it will not
push ra into the stack, such as lkdtm_BUG() function,

addi	sp,sp,-16
sd	s0,8(sp)
addi	s0,sp,16
ebreak

The struct stackframe use {fp,ra} to get information from
stack, if walk_stackframe() with pr_regs, we will obtain
wrong value and bad stacktrace,

[<ffffffe00066c56c>] lkdtm_BUG+0x6/0x8
---[ end trace 18da3fbdf08e25d5 ]---

Correct the next fp and pc, after that, full stacktrace
shown as expects,

[<ffffffe00066c56c>] lkdtm_BUG+0x6/0x8
[<ffffffe0008b24a4>] lkdtm_do_action+0x14/0x1c
[<ffffffe00066c372>] direct_entry+0xc0/0x10a
[<ffffffe000439f86>] full_proxy_write+0x42/0x6a
[<ffffffe000309626>] vfs_write+0x7e/0x214
[<ffffffe00030992a>] ksys_write+0x98/0xc0
[<ffffffe000309960>] sys_write+0xe/0x16
[<ffffffe0002014bc>] ret_from_syscall+0x0/0x2
---[ end trace 61917f3d9a9fadcd ]---
Signed-off-by: Chen Huang <chenhuang5@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/stacktrace.c

@@ -54,10 +54,16 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
 		/* Unwind stack frame */
 		frame = (struct stackframe *)fp - 1;
 		sp = fp;
+		if (regs && (regs->epc == pc) && (frame->fp & 0x7)) {
+			fp = frame->ra;
+			pc = regs->ra;
+		} else {
 			fp = frame->fp;
 			pc = ftrace_graph_ret_addr(current, NULL, frame->ra,
 						   (unsigned long *)(fp - 8));
 		}
+
+	}
 }
 
 #else /* !CONFIG_FRAME_POINTER */