osd/PrimaryLogPG: don't populate watchers if replica

osd/PrimaryLogPG: don't populate watchers if replica

If an object has an established watch, serving a read from replica
eventually leads to an assert in is_degraded_or_backfilling_object()
called from handle_watch_timeout().

The issue is that after can_serve_replica_read() check is satisfied,
we look up the object context.  If not found, we fetch the object info
and then call populate_obc_watchers() which sees the recorded watch and
treats it as unconnected, arming HandleWatchTimeout on replica.

Fixes: https://tracker.ceph.com/issues/45795
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>