crimson/osd/object_context_loader: Fix obc cache existence usage

crimson/osd/object_context_loader: Fix obc cache existence usage

with_head_obc() uses get_cached_obc() to get_or_create obc instances.

If the obc exists in cache, get_or_load_obc is called with `existed`=true.
The assumption above is wrong.
Cache existence (`existed`) only guarantees that the obc instance was created (and inserted) in the obc_registery.
However, it does **not** assure that the obc was actually loaded.

As obc-loading is now concurrent, it's possible for the first user to only create the obc in
cache (without loading yet) and the second concurrent user to assume it was already loaded.

With this patch, we verify that the obc was loaded in get_or_load_obc.

* make loading-obc concurrent PR: https://github.com/ceph/ceph/pull/55488

Fixes: https://tracker.ceph.com/issues/64206
Fixes: https://tracker.ceph.com/issues/66214
Signed-off-by: Matan Breizman <mbreizma@redhat.com>