LU-17945 lnet: fix nla_extract_val() -Werror=missing-prototypes

LU-17945 lnet: fix nla_extract_val() -Werror=missing-prototypes

This patch explicitly defines nla_extract_val() as a static function
to fix the following build failure:

lnet/lnet/api-ni.c:2888:1: error:
no previous prototype for 'nla_extract_val' [-Werror=missing-prototypes]
 2888 | nla_extract_val(struct nlattr **attr, int *rem,
      | ^~~~~~~~~~~~~~~

Test-Parameters: trivial testlist=sanity-lnet

Change-Id: Ieb11d25ea8fcd19b715e2decf958cfd9d920bcc8
Fixes: 629d80d ("LU-10003 lnet: migrate fail nid to Netlink")
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55413
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: Oleg Drokin <green@whamcloud.com>