tracing/ksym_tracer: fix write operation of ksym_trace_filter

This patch fix 2 bugs:
- fix the return value of ksym_trace_filter_write() when we want to
  clear symbol in ksym_trace_filter file
  for example:
  # echo global_trace:rw- > /debug/tracing/ksym_trace_filter
  # echo global_trace:--- > /debug/tracing/ksym_trace_filter
  -bash: echo: write error: Invalid argument
  # cat /debug/tracing/ksym_trace_filter
  #
  We want to clear 'global_trace' in ksym_trace_filter, it complain
  with "Invalid argument", but the operation is successful

- the "r--" access types is not allowed, but ksym_trace_filter file think
  it OK
  for example:
  # echo ksym_tracer_mutex:r-- > ksym_trace_filter
  -bash: echo: write error: Resource temporarily unavailable
  # dmesg
  ksym_tracer request failed. Try again later!!

  The error occur at register_kernel_hw_breakpoint(), but It's should
  at access types parser

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
LKML-Reference: <4A66863D.5090802@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_ksym.c

@@ -135,6 +135,9 @@ static int ksym_trace_get_access_type(char *str)
 	case 6:
 		access = HW_BREAKPOINT_RW;
 		break;
+	case 4:
+		access = -EINVAL;
+		break;
 	case 2:
 		access = HW_BREAKPOINT_WRITE;
 		break;
@@ -312,6 +315,7 @@ static ssize_t ksym_trace_filter_write(struct file *file,
 		kfree(entry->ksym_hbp->info.name);
 		kfree(entry->ksym_hbp);
 		kfree(entry);
+		ret = 0;
 		goto out;
 	} else {
 		/* Check for malformed request: (4) */