os/bluestore: prevent rename src from getting trimmed from lru

os/bluestore: prevent rename src from getting trimmed from lru

We were putting it at the end of the LRU *and* not holding a reference.

Fix by letting oldo carry a ref to the Onode.  This avoids a subsequent
lookup and also prevents it from getting trimmed.

Note that we could also put it at the front of the LRU, but given that it
is the rename source, I'm not sure that's appropriate.  For example,
temp recovery objects get renamed into place and we don't want to keep
negative onodes for those around.

Signed-off-by: Sage Weil <sage@redhat.com>