tracing/stat: Fix seqfile memory leak

Every time we cat a trace_stat file, we leak memory allocated by
seq_open().

Also fix memory leak in a failure path in tracing_stat_open().

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
LKML-Reference: <4A67D92B.4060704@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>