x86: Fix __get_wchan() for !STACKTRACE

Use asm/unwind.h to implement wchan, since we cannot always rely on
STACKTRACE=y.

Fixes: bc9bbb81 ("x86: Fix get_wchan() to support the ORC unwinder")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20211022152104.137058575@infradead.org

arch/x86/kernel/process.c

@@ -43,6 +43,7 @@
 #include <asm/io_bitmap.h>
 #include <asm/proto.h>
 #include <asm/frame.h>
+#include <asm/unwind.h>
 
 #include "process.h"
 
@@ -944,10 +945,20 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
  */
 unsigned long __get_wchan(struct task_struct *p)
 {
-	unsigned long entry = 0;
+	struct unwind_state state;
+	unsigned long addr = 0;
 
-	stack_trace_save_tsk(p, &entry, 1, 0);
-	return entry;
+	for (unwind_start(&state, p, NULL, NULL); !unwind_done(&state);
+	     unwind_next_frame(&state)) {
+		addr = unwind_get_return_address(&state);
+		if (!addr)
+			break;
+		if (in_sched_functions(addr))
+			continue;
+		break;
+	}
+
+	return addr;
 }
 
 long do_arch_prctl_common(struct task_struct *task, int option,