umem: don't override the queue_lock

The umem card->lock and the block layer queue_lock are used for entirely
different resources.  Stop using card->lock as the block layer
queue_lock.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>