block: split block_class_lock

Split the block_class_lock mutex into one each to protect bdev_map
and major_names.

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