blktrace: pass the right pointer to kfree()

Impact: fix kfree crash with non-standard act_mask string

If passing a string with leading white spaces to strstrip(),
the returned ptr != the original ptr.

This bug was introduced by me.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <49DD694C.8020902@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/blktrace.c

@@ -1377,12 +1377,12 @@ static int blk_trace_str2mask(const char *str)
 {
 	int i;
 	int mask = 0;
-	char *s, *token;
+	char *buf, *s, *token;
 
-	s = kstrdup(str, GFP_KERNEL);
-	if (s == NULL)
+	buf = kstrdup(str, GFP_KERNEL);
+	if (buf == NULL)
 		return -ENOMEM;
-	s = strstrip(s);
+	s = strstrip(buf);
 
 	while (1) {
 		token = strsep(&s, ",");
@@ -1403,7 +1403,7 @@ static int blk_trace_str2mask(const char *str)
 			break;
 		}
 	}
-	kfree(s);
+	kfree(buf);
 
 	return mask;
 }