ceph: make f_bsize always equal to f_frsize

The f_frsize maybe changed in the quota size is less than the defualt
4MB.

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