ceph: implement readv/preadv for sync operation

For readv/preadv sync-operation, ceph only does the first iov.
Now implement this.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Reviewed-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Sage Weil <sage@inktank.com>