LU-16847 ldiskfs: refactor code brw_stats code.

LU-16847 ldiskfs: refactor code brw_stats code.

counting a number disk or logical extents don't
needs a loop.
All information exist around of ldiskfs_map_blocks.

HPe-bug-id: LUS-11645
Signed-off-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Change-Id: I77f3707b88e9bdf6ea06acc950af2a41f056f5d0
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51391
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>