block: put dev->kobj in blk_register_queue fail path

kernel needs to kobject_put on dev->kobj if elv_register_queue fails.

Signed-off-by: Xiaotian Feng <dfeng@redhat.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Nikanth Karthikesan <knikanth@suse.de>
Cc: David Teigland <teigland@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>