ReplicatedPG: CEPH_OSD_OP_WATCH return -ENOENT if !obs.exists

Fixes: #7671
Signed-off-by: Samuel Just <sam.just@inktank.com>
test/system/st_rados_watch: create object before watching

Signed-off-by: Samuel Just <sam.just@inktank.com>
Revert "ReplicatedPG: CEPH_OSD_OP_WATCH should just create the object"

This reverts commit c79ef75620f9e30675c234ae0a4e8fb58f1affe4.
Merge remote-tracking branch 'origin/wip-7671' into wip-sam-testing