rbd: combine rbd sync watch/unwatch functions

The rbd_req_sync_watch() and rbd_req_sync_unwatch() functions are
nearly identical.  Combine them into a single function with a flag
indicating whether a watch is to be initiated or torn down.

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