MIPS: ftrace: Fix icache flush range error

In 32-bit mode, the start address passed to flush_icache_range is
shifted by 4 bytes before the second safe_store_code() call.

This causes system crash from time to time because the first 4 bytes
might not be flushed properly. This bug exists since linux-3.8.

Also remove obsoleted comment while at it.

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: rostedt@goodmis.org
Cc: fweisbec@gmail.com
Cc: mingo@redhat.com
Cc: Qais.Yousef@imgtec.com
Patchwork: https://patchwork.linux-mips.org/patch/6586/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/ftrace.c

@@ -111,11 +111,10 @@ static int ftrace_modify_code_2(unsigned long ip, unsigned int new_code1,
 	safe_store_code(new_code1, ip, faulted);
 	if (unlikely(faulted))
 		return -EFAULT;
-	ip += 4;
-	safe_store_code(new_code2, ip, faulted);
+	safe_store_code(new_code2, ip + 4, faulted);
 	if (unlikely(faulted))
 		return -EFAULT;
-	flush_icache_range(ip, ip + 8); /* original ip + 12 */
+	flush_icache_range(ip, ip + 8);
 	return 0;
 }
 #endif