os/bluestore: allow reuse of osr from existing collection

os/bluestore: allow reuse of osr from existing collection

We try to attach an old osr at prepare_new_collection time, but that
happens before a transaction is submitted, and we might have a
transaction that removes and then recreates a collection.

Move the logic to _osr_attach and extend it to include reusing an osr
in use by a collection already in coll_map.  Also adjust the
_osr_register_zombie method to behave if the osr is already there, which
can happen with a remove, create, remove+create transaction sequence.

Fixes: https://tracker.ceph.com/issues/25180
Signed-off-by: Sage Weil <sage@redhat.com>