ceph: fix dir_lease_is_valid()

It should call __ceph_dentry_dir_lease_touch() under dentry->d_lock.
Besides, ceph_dentry(dentry) can be NULL when called by LOOKUP_RCU
d_revalidate()

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