/proc/fs/jbd2/${device}/info only shows whole average statistical
info about jbd2's life cycle, but it can not show jbd2 info in
specified time interval and sometimes this capability is very useful
for trouble shooting. For example, we can not see how rs_locked and
rs_flushing grows in specified time interval, but these two indexes
can explain some reasons for app's behaviours.

Here we add a new "stats" proc file like /proc/diskstats, then we can
implement a simple tool jbd2_stats which'll display detailed jbd2 info
in specified time interval. Like below(time interval 5s):

[lege@localhost ~]$ cat /proc/fs/jbd2/vdb1-8/stats
51 30 8192 0 1 241616 0 0 22 0 47158 891 942 1000 1000

[lege@localhost ~]$ gcc -o jbd2_stat jbd2_stat.c ; ./jbd2_stat

Device              tid     trans   handles    locked  flushing
logging
vdb1-8             1861       158       359     13.00      0.00
2.00

Device              tid     trans   handles    locked  flushing
logging
vdb1-8             1974       113       389     26.00      0.00
5.00

Device              tid     trans   handles    locked  flushing
logging
vdb1-8             2188       214       308     10.00      0.00
7.00

Device              tid     trans   handles    locked  flushing
logging
vdb1-8             2344       156       332     19.00      0.00
4.00
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NLiu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>