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>
(cherry picked from commit cdb7675a21c9107e3596c90c2b1598def3c6899f)
Merge pull request #2494 from ceph/wip-rbd-objectcacher-hang-giant

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