rbd: introduce rbd_dev_header_unwatch_sync() and switch to it

Rename rbd_dev_header_watch_sync() to __rbd_dev_header_watch_sync() and
introduce two helpers: rbd_dev_header_{,un}watch_sync() to make it more
clear what is going on.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
rbd: tear down watch request if rbd_dev_device_setup() fails

Tear down watch request if rbd_dev_device_setup() fails.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>