ceph: don't update_dentry_lease unless we actually got one

This if block updates the dentry lease even in the case where
the MDS didn't grant one.

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