block: take bd_mutex around delete_partitions in del_gendisk

There is nothing preventing an ioctl from trying do delete partition
concurrenly with del_gendisk, so take open_mutex to serialize against
that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210406062303.811835-6-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>