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: NIlya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>