LU-14131 osd-ldiskfs: reduce credits for overwritting

LU-14131 osd-ldiskfs: reduce credits for overwritting

If all blocks are mapped which means this is overwritting
case or space has been allocated by fallocate.

There is no need to modify exten tree, and we only
need 1 credits for inode.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I907861d4862256a8a23a81812953e2330e1d9925
Reviewed-on: https://review.whamcloud.com/40604
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>