bluestore: do not crash on over-large objects
See "[ceph-users] Getting errors on erasure pool writes k=2, m=1" for one example. We've had a number of reports from users putting in overly-large objects and causing their OSDs to crash.
Obviously it would be good if we prevented them doing this from higher layers (...which I thought we did, now?), but we also need BlueStore to start rejecting IOs instead of crashing.