riscv: stacktrace: Remove bogus -0x4 offset in non-FP walk_stackframe

In the non-frame-pointer version of walk_stackframe, each value read
from the stack is treated as a potential return address and has 0x4
subtracted before being used as the program counter. This was intended
to convert the return address (the instruction after a call) back to
the call site, but it is incorrect:

1. RISC-V has variable-length instructions due to the RVC (compressed
   instruction) extension. A call instruction can be either 4 bytes
   (regular) or 2 bytes (compressed, e.g. c.jal). Subtracting a fixed
   0x4 assumes all call instructions are 4 bytes, which is wrong for
   compressed instructions.

2. Stack traces conventionally report return addresses, not call sites.
   Other architectures (ARM64, x86, ARM) do not subtract instruction
   size from return addresses in their stack unwinding code.

3. The frame-pointer version of walk_stackframe already dropped the
   -0x4 offset. Commit b785ec129b ("riscv/ftrace: Add
   HAVE_FUNCTION_GRAPH_RET_ADDR_PTR support") replaced "pc =
   frame->ra - 0x4" with ftrace_graph_ret_addr(), and the commit
   message explicitly noted that "the original calculation, pc =
   frame->ra - 4, is buggy when the instruction at the return address
   happened to be a compressed inst." The non-FP version was simply
   overlooked.

Remove the bogus -0x4 offset to match the FP version and the
conventions used by other architectures.

Fixes: 5d8544e2d0 ("RISC-V: Generic library routines and assembly")
Signed-off-by: Rui Qi <qirui.001@bytedance.com>
Link: https://patch.msgid.link/20260603115329.791603-2-qirui.001@bytedance.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>

arch/riscv/kernel/stacktrace.c

@@ -129,7 +129,7 @@ void notrace walk_stackframe(struct task_struct *task,
 	while (!kstack_end(ksp)) {
 		if (__kernel_text_address(pc) && unlikely(!fn(arg, pc)))
 			break;
-		pc = READ_ONCE_NOCHECK(*ksp++) - 0x4;
+		pc = READ_ONCE_NOCHECK(*ksp++);
 	}
 }