LU-14918 osd: don't declare similar ldiskfs writes twice

LU-14918 osd: don't declare similar ldiskfs writes twice

in some cases (like overstriping) the same operations can be
declared multiple times (new llog records) and this lead to
huge number of credits and performance degradation. we can
avoid this checking for duplicate declarations.
As every declaration would need an allocation, limit the scope
of this checks to transaction likely to be large.

% of "large" transaction in sanity-benchmark, depending on threshold:

  creates < 5 && writes < 5:
  0.58% (mds1) and  2.97% (mds2)

  create < 7 & writes < 7:
  0.58% and 2.4%

  create < 9 & writes  < 9:
  0.6% and 1.85%

  create < 10 & write2 < 10:
  0.0004% and 0.000001%

thus 10 creates or writes is selected as a threshold to enable this
logic.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I7c893fe3b95646b4b813b999bc832659dfcf03ad
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/45765
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>