crimson/osd: return ENOENT from EC getxattr for non-existent objects

crimson/osd: return ENOENT from EC getxattr for non-existent objects

PGBackend::getxattr() for EC pools reads from the attr_cache and
returns ENODATA when an attr is not found, without distinguishing
between "object exists but attr not set" and "object doesn't exist".
This causes cls methods like cls_refcount_get() to proceed as if the
object exists, enter the EC RMW pipeline, and hang.

Add an os.exists check in the EC path so non-existent objects return
ENOENT, matching the behavior of the replicated path which goes to
the object store and gets ENOENT directly.

Fixes: https://tracker.ceph.com/issues/77335
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>