ReplicatedPG: cancel cb on blacklisted watcher

Fixes: #8315
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2522 from athanatos/wip-8315

ReplicatedPG: cancel cb on blacklisted watcher

Reviewed-by: Sage Weil <sage@redhat.com>