LU-12452 lnet: inherit default lnd tunings from modparams

LU-12452 lnet: inherit default lnd tunings from modparams

When a network is added dynamically (via Netlink), LNet assumes that
all the "unset" or default LND parameters are 0. But for some
use cases, 0 could be a valid value.

This patch modifies the callback lnet_lnd.lnd_nl_set() to set default
values: a NULL attr argument means "set the default value".

Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: Ifb91ae63d96131ed87d9fae7d91b8b18df4c9ce9
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54078
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>