ReplicatedPG: record correctly subop for perfcounter.

In log_subop_stats, it omit to record the counter of subop.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>
ReplicatedPG: Make perfcounter record the read-size for
 async-read.

Signed-off-by: Ma Jianpeng <jianpeng.ma@intel.com>