libceph: add osd_req_op_extent_osd_data_bvecs()

... and store num_bvecs for client code's convenience.

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