ksym_tracer: Fix to allow writing newline to ksym_trace_filter

It used to work, but now doesn't:

 # echo > ksym_filter
 bash: echo: write error: Invalid argument

It's caused by d954fbf0ff6b5fdfb32350e85a2f15d3db976506
("tracing: Fix wrong usage of strstrip in trace_ksyms").

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: K.Prasad <prasad@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <4B3AF1D7.5040400@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_ksym.c

@@ -299,8 +299,8 @@ static ssize_t ksym_trace_filter_write(struct file *file,
 	 * 2: echo 0 > ksym_trace_filter
 	 * 3: echo "*:---" > ksym_trace_filter
 	 */
-	if (!buf[0] || !strcmp(buf, "0") ||
-	    !strcmp(buf, "*:---")) {
+	if (!input_string[0] || !strcmp(input_string, "0") ||
+	    !strcmp(input_string, "*:---")) {
 		__ksym_trace_reset();
 		ret = 0;
 		goto out;