ReplicatedPG: return no data if read size is trimmed to zero

OSD should return no data if the read size is trimmed to zero by the
truncate_seq/truncate_size check. We can't rely on ObjectStore::read()
to do that because it reads the entire object when the 'len' parameter
is zero.

Fixes: #7371
Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #1251 from ceph/wip-7371

ReplicatedPG: return no data if read size is trimmed to zero

Backport: emperor, dumpling
Reviewed-by: Sage Weil <sage@inktank.com>