osd: Make RPGTransaction::get_bytes_written return the correct size.

It record size larger than clien wrote. It should like
ECTransaction::get_bytes_written only return the size which clien
wrote. It should contain omap data.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
Merge pull request #2400 from majianpeng/fix2

osd: Make RPGTransaction::get_bytes_written return the correct size.

Reviewed-by: Sage Weil <sage@redhat.com>