f2fs: allow to read node block after shutdown

If block address is still alive, we should give a valid node block even after
shutdown. Otherwise, we can see zero data when reading out a file.

Cc: stable@vger.kernel.org
Fixes: 83a3bfdb

 ("f2fs: indicate shutdown f2fs to allow unmount successfully")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>