libceph: change ceph_osdc_call() to take page vector for response

This will be used for loading object map.  rbd_obj_read_sync() isn't
suitable because object map must be accessed through class methods.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
Reviewed-by: Jeff Layton <jlayton@kernel.org>