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.

Lustre-change: https://review.whamcloud.com/44316
Lustre-commit: 934758366dbde211743d53b8c048c57819603cb3

Fixes: 9810341a8 ("LU-14729 osd-ldiskfs: fix to declare write commits")
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I1e0fc9069a579736a74b0ba2607056fe980574c3
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/45077
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>