LU-14641 osd-ldiskfs: write commit declaring improvement

This patch try to:

1)extent bytes could be missed to increase with less than
1M, fix to to compare it with current value, and decay
it for every allocation.

2)with system space usage growing up, mballoc codes won't
try best to scan block group to align best free extent as
we can. So extent bytes per extent could be decayed to a
very small value, this could make us reserve too many credits.
We could be more optimistic in the credit reservations, even
in a case where the filesystem is nearly full, it is extremely
unlikely that the worst case would ever be hit.

3)Add extent bytes stats and debug ability to analysis
over reservation problem.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I357c4a855147ba26a9e9bbe9ab1269bcfd44e5f3
Reviewed-on: https://review.whamcloud.com/43446
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>