LU-14729 osd-ldiskfs: declare should consider concurrency

LU-14729 osd-ldiskfs: declare should consider concurrency

Write in Lustre OSD is different than Ext4 since write
is serialized in local filesystem, however in OSD side,

many concurrent threads may grow tree before transaction starts.

Also fix to use @dirty_groups rather than @extents, remove
unnecessary @depth assignment.

Fixes: 9810341a8 ("LU-14729 osd-ldiskfs: fix to declare write commits")
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I1e0fc9069a579736a74b0ba2607056fe980574c3
Reviewed-on: https://review.whamcloud.com/44316
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>