Blktrace: bail out early if block debugfs is not configured

Blktrace: bail out early if block debugfs is not configured

commit e1a413245a564683697a3d02ec197b72cf009b89 upstream.

Since @blk_debugfs_root couldn't be configured dynamically, we can
save a few memory allocation if it's not there.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[bwh: Cherry-picked for 4.14 to ease backporting a later fix]
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>