LoongArch: Get partial stack information when providing regs parameter

Currently, arch_stack_walk() can only get the full stack information
including NMI.  This is because the implementation of arch_stack_walk()
is forced to ignore the information passed by the regs parameter and use
the current stack information instead.

For some detection systems like KFENCE, only partial stack information
is needed.  In particular, the stack frame where the interrupt occurred.

To support KFENCE, this patch modifies the implementation of the
arch_stack_walk() function so that if this function is called with the
regs argument passed, it retains all the stack information in regs and
uses it to provide accurate information.

Before this patch:
[    1.531195 ] ==================================================================
[    1.531442 ] BUG: KFENCE: out-of-bounds read in stack_trace_save_regs+0x48/0x6c
[    1.531442 ]
[    1.531900 ] Out-of-bounds read at 0xffff800012267fff (1B left of kfence-#12):
[    1.532046 ]  stack_trace_save_regs+0x48/0x6c
[    1.532169 ]  kfence_report_error+0xa4/0x528
[    1.532276 ]  kfence_handle_page_fault+0x124/0x270
[    1.532388 ]  no_context+0x50/0x94
[    1.532453 ]  do_page_fault+0x1a8/0x36c
[    1.532524 ]  tlb_do_page_fault_0+0x118/0x1b4
[    1.532623 ]  test_out_of_bounds_read+0xa0/0x1d8
[    1.532745 ]  kunit_generic_run_threadfn_adapter+0x1c/0x28
[    1.532854 ]  kthread+0x124/0x130
[    1.532922 ]  ret_from_kernel_thread+0xc/0xa4
<snip>

After this patch:
[    1.320220 ] ==================================================================
[    1.320401 ] BUG: KFENCE: out-of-bounds read in test_out_of_bounds_read+0xa8/0x1d8
[    1.320401 ]
[    1.320898 ] Out-of-bounds read at 0xffff800012257fff (1B left of kfence-#10):
[    1.321134 ]  test_out_of_bounds_read+0xa8/0x1d8
[    1.321264 ]  kunit_generic_run_threadfn_adapter+0x1c/0x28
[    1.321392 ]  kthread+0x124/0x130
[    1.321459 ]  ret_from_kernel_thread+0xc/0xa4
<snip>
Suggested-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Enze Li <lienze@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/kernel/stacktrace.c

@@ -18,17 +18,19 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
 	struct pt_regs dummyregs;
 	struct unwind_state state;
 
-	regs = &dummyregs;
+	if (!regs) {
+		regs = &dummyregs;
 
-	if (task == current) {
-		regs->regs[3] = (unsigned long)__builtin_frame_address(0);
-		regs->csr_era = (unsigned long)__builtin_return_address(0);
-	} else {
-		regs->regs[3] = thread_saved_fp(task);
-		regs->csr_era = thread_saved_ra(task);
+		if (task == current) {
+			regs->regs[3] = (unsigned long)__builtin_frame_address(0);
+			regs->csr_era = (unsigned long)__builtin_return_address(0);
+		} else {
+			regs->regs[3] = thread_saved_fp(task);
+			regs->csr_era = thread_saved_ra(task);
+		}
+		regs->regs[1] = 0;
 	}
 
-	regs->regs[1] = 0;
 	for (unwind_start(&state, task, regs);
 	     !unwind_done(&state) && !unwind_error(&state); unwind_next_frame(&state)) {
 		addr = unwind_get_return_address(&state);