rbd: drop dev registration check for new snap

By the time rbd_dev_snaps_register() gets called during rbd device
initialization, the main device will have already been registered.
Similarly, a header refresh will only occur for an rbd device whose
Linux device is registered.  There is therefore no need to verify
the main device is registered when registering a snapshot device.

For the time being, turn the check into a WARN_ON(), but it can
eventually just go away.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>