blktrace: remove debugfs file dentries from struct blk_trace

blktrace: remove debugfs file dentries from struct blk_trace

These debugfs dentries do not need to be saved for anything as the whole
directory and everything in it is properly cleaned up when the parent
directory is removed.  So remove them from struct blk_trace and don't
save them when created as it's not necessary.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: linux-block@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>