LU-13903 utils: move userland only nidstr.h handling

LU-13903 utils: move userland only nidstr.h handling

The function cfs_expand_nidlist() no longer exist for kernel
internals. We can move the function prototype from the UAPI
header to string.h which is a libcfs user land header.
The structure netstrfns that is defined in a UAPI header
has been adding user land only handling. Additional its
use struct list_head which will confuse reviewers since
kernel developers see this as a kernel only thing.

Lustre-change: https://review.whamcloud.com/39115
Lustre-commit: 062809b1313ac7000e3476c96cbbe5d017f10875

Test-Parameters: trivial

Change-Id: Ifc3c87f6d3237a94d282d009455ff389278e73ea
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/49777
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>