block: grab a device refcount in disk_uevent

Sending uevents requires the struct device to be alive.  To
ensure that grab the device refcount instead of just an inode
reference.

Fixes: bc359d03

 ("block: add a disk_uevent helper")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210701081638.246552-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>