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.

Test-parameters: trivial

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: Iffc7a128b16bc1b2be7a44413a5972c97b12a5fa
Reviewed-on: https://review.whamcloud.com/43525
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>