blk-mq: fix up placement of debugfs directory of queue files

When the blk-mq debugfs file creation logic was "cleaned up" it was
cleaned up too much, causing the queue file to not be created in the
correct location.  Turns out the check for the directory being present
is needed as if that has not happened yet, the files should not be
created, and the function will be called later on in the initialization
code so that the files can be created in the correct location.

Fixes: 6cfc0081 ("blk-mq: no need to check return value of debugfs_create functions")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: linux-block@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>