blktrace: fix possible memory leak

When we failed to create "block" debugfs dir, we should do some
cleanups.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <49C2F5B2.8000800@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>