Both ceph_sync_direct_write and ceph_sync_read iterate iovec elements
one by one, send one OSD request for each iovec. This is sub-optimal,
We can combine serveral iovec into one page vector, and send an OSD
request for the whole page vector.
Signed-off-by: NZhu, Caifeng <zhucaifeng@unissoft-nj.com>
Signed-off-by: NYan, Zheng <zyan@redhat.com>