LU-10003 lnet: to properly handle errors reported use ssize_t

LU-10003 lnet: to properly handle errors reported use ssize_t

The function nla_strscpy() can return a negative error value.
Replace using size_t with ssize_t to handle potential error
states.

Test-Parameters: trivial
Fixes: 8f8f6e2f3 ("LU-10003 lnet: use Netlink to support old and new NI APIs.")
Change-Id: I079f93e3e1005a1062a3fe0807cf8b877712d818
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49068
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>