ceph: refactor ceph_sync_read()

Avoid allocating memory for the entire user request: striped_read()
does a synchronous OSD request per object, so it doesn't need more than
object size worth of pages at a time.

[ Preserve the comment, changelog. ]

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>