ceph: do a LOOKUP in d_revalidate instead of GETATTR

ceph: do a LOOKUP in d_revalidate instead of GETATTR

In commit c3f4688a08f (ceph: don't set req->r_locked_dir in
ceph_d_revalidate), we changed the code to do a GETATTR instead of a
LOOKUP as the parent info isn't strictly necessary to revalidate the
dentry. What we missed there though is that in order to update the lease
on the dentry after revalidating it, we _do_ need parent info.

Change ceph_d_revalidate back to doing a LOOKUP instead of a GETATTR so
that we can get the parent info in order to update the lease from
ceph_fill_trace. Note that we set req->r_parent here, but we cannot set
the CEPH_MDS_R_PARENT_LOCKED flag as we can't guarantee that it is.

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