ceph: do not drop lease during revalidate

We need to hold session s_mutex for __ceph_mdsc_drop_dentry_lease(), which
we don't, so skip it.  It was purely an optimization.
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/dir.c

@@ -920,8 +920,6 @@ static int dentry_lease_is_valid(struct dentry *dentry)
 				di->lease_renew_after = 0;
 				di->lease_renew_from = jiffies;
 			}
-		} else {
-			__ceph_mdsc_drop_dentry_lease(dentry);
 		}
 	}
 	spin_unlock(&dentry->d_lock);