Fix the following kernel-doc warning:

include/linux/jbd2.h:1184: warning: Function parameter or member 'j_checkpoint_task' not described in 'journal_s'

Fixes: c31b17e5 ("alios: jbd2: create jbd2-ckpt thread for journal checkpoint")
Reported-by: Nkbuild test robot <lkp@intel.com>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NJeffle Xu <jefflexu@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>