ceph: Implement writev/pwritev for sync operation.
For writev/pwritev sync-operatoin, ceph only do the first iov.
I divided the write-sync-operation into two functions. One for
direct-write, other for none-direct-sync-write. This is because for
none-direct-sync-write we can merge iovs to one. But for direct-write,
we can't merge iovs.
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>