ReplicatedPG: For async-read, set the real result after completing read.

When reading an object from replicated pool, ceph uses sync mode,
so it can set the results in execute_ctx correctly.
However, For the async-read in EC Pool, current code didn't set the
real results after read in complete_read_ctx.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
Merge pull request #2175 from majianpeng/fix1

ReplicatedPG: For async-read, set the real result after completing read.

Reviewed-by: Samuel Just <sam.just@inktank.com>