ceph: fix description of some mount options

Based on the latest code, the default value for wsize/rsize is
64MB and the default value for the mount_timeout is 60 seconds.

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