LU-17471 osd-ldiskfs: brw_stats symlink module option

LU-17471 osd-ldiskfs: brw_stats symlink module option

Add a module option to symlink /proc/fs/lustre/osd-*/*/brw_stats
to /sys/kernel/debug/*/*/brw_stats for backward compatibility
for systems that need this for monitoring tools that depend on the
old /proc pathname instead of always creating it at startup.

Otherwise, "lctl get_param osd-ldiskfs.*.brw_stats" will display
the stats for both the original and symlinked parameter file,
which can confuse other monitoring tools.

To enable the /proc/.../brw_stats symlink, add the following line
into /etc/modprobe.d/lustre.conf before loading modules/mounting:

  options osd_ldiskfs symlink_brw_stats=1

By default the symlink is not created, since newer tools should
be using the /sys/kernel/debug/.../brw_stats parameter file.

Remove sanity test_0f that is verifying the old parameter path,
since it is just too messy to get the interoperability correct.

Test-Parameters: trivial
Fixes: 47ccacd58d ("LU-17471 osd: add symlink for brw_stats")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ib61630cf2bc89b84af438c811754a85e403ebbe5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56917
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>