LU-17471 osd: add symlink for brw_stats

LU-17471 osd: add symlink for brw_stats

Add symlink at /proc/fs/lustre/osd-*/*/brw_stats to
/sys/kernel/debug/lustre/osd-*/*/brw_stats to fix
the compatible issue of the previous utils that are
still using the old proc entry.

Test-Parameters: testlist=sanity env=ONLY=0f serverversion=2.15.4
Fixes: 8a84c7f9c7d6 ("LU-14927 osd: share brw_stats code between OSD back ends.")
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Change-Id: Ie86b2b384e3b91f98ead00b6325ddeb020e47aa5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53829
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>