LU-16959 lnet: auto-tune ARP-related sysctl setting

LU-16959 lnet: auto-tune ARP-related sysctl setting

Default linux settings for net.ipv4.neigh.default.gc_thresh* may be
too low. The configuration file contains recommended threshold values
for the arp table configuration for larger systems. These values are
not set by default and can be enabled by setting the
enable_sysctl_setup parameter to 1 in the configuration file.
To activate the changes immediately please execute
sysctl -p /etc/lnet-sysctl.conf as root.
New ticket fot documentation
LUDOC-528 - Adding documentation for enable_sysctl_setup

Test-Parameters: trivial testlist=sanity-lnet env=ONLY=260
Signed-off-by: Frank Sehr <fsehr@whamcloud.com>
Change-Id: I34af4b402b59341ee7e9cfb45fef7c67eb5e78e9
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53310
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>