s390/syscalls: Fix out of bounds arguments access

Zorro reported following crash while having enabled
syscall tracing (CONFIG_FTRACE_SYSCALLS):

  Unable to handle kernel pointer dereference at virtual ...
  Oops: 0011 [#1] SMP DEBUG_PAGEALLOC

  SNIP

  Call Trace:
  ([<000000000024d79c>] ftrace_syscall_enter+0xec/0x1d8)
   [<00000000001099c6>] do_syscall_trace_enter+0x236/0x2f8
   [<0000000000730f1c>] sysc_tracesys+0x1a/0x32
   [<000003fffcf946a2>] 0x3fffcf946a2
  INFO: lockdep is turned off.
  Last Breaking-Event-Address:
   [<000000000022dd44>] rb_event_data+0x34/0x40
  ---[ end trace 8c795f86b1b3f7b9 ]---

The crash happens in syscall_get_arguments function for
syscalls with zero arguments, that will try to access
first argument (args[0]) in event entry, but it's not
allocated.

Bail out of there are no arguments.

Cc: stable@vger.kernel.org
Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/syscall.h

@@ -64,6 +64,12 @@ static inline void syscall_get_arguments(struct task_struct *task,
 {
 	unsigned long mask = -1UL;
 
+	/*
+	 * No arguments for this syscall, there's nothing to do.
+	 */
+	if (!n)
+		return;
+
 	BUG_ON(i + n > 6);
 #ifdef CONFIG_COMPAT
 	if (test_tsk_thread_flag(task, TIF_31BIT))