[PATCH 1/2 RFC] srcu: change the comments of the wait algorithm

[Lai Jiangshan]

Hi, ALL

The call_srcu() will be sent soon(may be in 2 days). I found something is not
good in current sruc when I implement it, so I do more prepare for it.

The second patch is inspired Peter. I had decided to use per-cpu machine,
the the snap array makes me unhappy. If a machine is sleeping/preempted
while checking, the other machine can't not check the same srcu_struct.
It is nothing big, but it also blocks the sychronize_srcu_expedited().
I hope sychronize_srcu_expedited() can't be blocked when it try to do its
fast-checking. So I try to find non-block checking algorithm, and I find
Peter's.

The most things in these two patches are comments, so I bring a lot
troubles to Paul because my poor English.

Thanks,
Lai