libceph: define struct ceph_sparse_extent and add some helpers

libceph: define struct ceph_sparse_extent and add some helpers

When the OSD sends back a sparse read reply, it contains an array of
these structures. Define the structure and add a couple of helpers for
dealing with them.

Also add a place in struct ceph_osd_req_op to store the extent buffer,
and code to free it if it's populated when the req is torn down.

Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>