rbd: avoid format-security warning inside alloc_workqueue()

drivers/block/rbd.c: In function ‘rbd_dev_device_setup’:
drivers/block/rbd.c:5090:19: warning: format not a string literal and no format arguments [-Wformat-security]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>