rbd: unregister linger in watch sync routine

Move the code that unregisters an rbd device's lingering header
object watch request into rbd_dev_header_watch_sync(), so it
occurs in the same function that originally sets up that request.

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