osdc/ObjectCacher: overwrite might cause stray read request callbacks

osdc/ObjectCacher: overwrite might cause stray read request callbacks

In librbd, if readahead is active, there might be a pending read request
for the cache which is then (partially) overwritten by a write request.
This overwrite will cause bh splits and merges which can cause the
bh read callback to fail to invoke the pending read callbacks.

Fixes: https://tracker.ceph.com/issues/46822
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 94d43165ed7319d163640f38d154f8f80408eb14)