LU-17976 build: fix nla_strnid() -Werror=missing-prototypes

LU-17976 build: fix nla_strnid() -Werror=missing-prototypes

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

  lnet/lnet/api-ni.c:2929:1: error:
  no previous prototype for 'nla_strnid' [-Werror=missing-prototypes]
   2929 | nla_strnid(struct nlattr **attr, struct lnet_nid *nid, int *rem,
        | ^~~~~~~~~~

It also fixes:
- lnet_fault_show_done()
- lnet_fault_show_start()
- lnet_fault_show_dump()
- __ll_dio_user_copy()
- ll_dio_user_copy_helper()

Change-Id: I0225794b3fac2f36aafadff783ca921fbc757edd
Fixes: f1c6623 ("LU-10391 lnet: Fault injection add/del ioctls to netlink")
Fixes: 1fa633c ("LU-17478 clio: parallelize unaligned DIO write copy")
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55537
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>