librados: fix MWatchNotify leak

Do not leak the message if the watcher is not registered.  Also, simplify
this block.

Fixes (part of): #5949
Backport: dumpling, cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 6f5d8036f3e70c5e30edf7e36fb8ff9a56197f60)