mds: only open non-regular inode with mode FILE_MODE_PIN

ceph_atomic_open() in kernel client does lookup and open at the same
time. So it can open a symlink inode with mode CEPH_FILE_MODE_WR.
Open a symlink inode with mode CEPH_FILE_MODE_WR triggers assertion
in Locker::check_inode_max_size();

Signed-off-by: Yan, Zheng <zyan@redhat.com>
(cherry picked from commit 4d15eb1)

src/mds/Server.cc

@@ -2732,8 +2732,8 @@ void Server::handle_client_open(MDRequestRef& mdr)
     return;
   }
 
-  // can only open a dir with mode FILE_MODE_PIN, at least for now.
-  if (cur->inode.is_dir())
+  // can only open non-regular inode with mode FILE_MODE_PIN, at least for now.
+  if (!cur->inode.is_file())
     cmode = CEPH_FILE_MODE_PIN;
 
   dout(10) << "open flags = " << flags