LU-14665 lnet: simplify lnet_ni_add_interface

LU-14665 lnet: simplify lnet_ni_add_interface

Remove an unnecessary counter and move the comment before
the relevant code.  Improve error messages.

Lustre-change: https://review.whamcloud.com/43525
Lustre-commit: b77a6d86936c32bb5f36e9806323ba00a18b0f4b

Test-parameters: trivial

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: Iffc7a128b16bc1b2be7a44413a5972c97b12a5fa
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/54568
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>