s390/process: avoid custom stack unwinding in get_wchan

Currently get_wchan uses custom stack unwinding implementation which
relies on back_chain presence. Replace it with more abstract stack
unwinding api usage.
Suggested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/process.c

@@ -40,6 +40,7 @@
 #include <asm/stacktrace.h>
 #include <asm/switch_to.h>
 #include <asm/runtime_instr.h>
+#include <asm/unwind.h>
 #include "entry.h"
 
 asmlinkage void ret_from_fork(void) asm ("ret_from_fork");
@@ -178,9 +179,8 @@ EXPORT_SYMBOL(dump_fpu);
 
 unsigned long get_wchan(struct task_struct *p)
 {
-	struct stack_frame *sf, *low, *high;
-	unsigned long return_address;
-	int count;
+	struct unwind_state state;
+	unsigned long ip = 0;
 
 	if (!p || p == current || p->state == TASK_RUNNING || !task_stack_page(p))
 		return 0;
@@ -188,26 +188,22 @@ unsigned long get_wchan(struct task_struct *p)
 	if (!try_get_task_stack(p))
 		return 0;
 
-	low = task_stack_page(p);
-	high = (struct stack_frame *) task_pt_regs(p);
-	sf = (struct stack_frame *) p->thread.ksp;
-	if (sf <= low || sf > high) {
-		return_address = 0;
-		goto out;
+	unwind_for_each_frame(&state, p, NULL, 0) {
+		if (state.stack_info.type != STACK_TYPE_TASK) {
+			ip = 0;
+			break;
 		}
-	for (count = 0; count < 16; count++) {
-		sf = (struct stack_frame *)READ_ONCE_NOCHECK(sf->back_chain);
-		if (sf <= low || sf > high) {
-			return_address = 0;
-			goto out;
-		}
-		return_address = READ_ONCE_NOCHECK(sf->gprs[8]);
-		if (!in_sched_functions(return_address))
-			goto out;
+
+		ip = unwind_get_return_address(&state);
+		if (!ip)
+			break;
+
+		if (!in_sched_functions(ip))
+			break;
 	}
-out:
+
 	put_task_stack(p);
-	return return_address;
+	return ip;
 }
 
 unsigned long arch_align_stack(unsigned long sp)