rbd: ObjectCacher reads can hang when reading sparse files

The pending read list was not properly flushed when empty objects
were read from a space file.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Merge pull request #2493 from ceph/wip-rbd-objectcacher-hang

rbd: ObjectCacher reads can hang when reading sparse files

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>