ObjectCacher: fix bh_read_finish offset logic

ObjectCacher: fix bh_read_finish offset logic

If we have an incoming read split across multiple BufferHeads, we want to
line up the BufferHead's bl with the incoming OSDOp's bl at the right offset. We
were erroneously using this nonsense calculation (always equal to zero!) when
a much simpler comparison of the BufferHead's logical object offset to the
incoming OSDOp's logical offset will do the trick nicely.

Fixes: http://tracker.ceph.com/issues/16002
Signed-off-by: Greg Farnum <gfarnum@redhat.com>
(cherry picked from commit 9ec6e7f608608088d51e449c9d375844631dcdde)