LU-12626 lnet: create existing net returns EEXIST

LU-12626 lnet: create existing net returns EEXIST

When "lnetctl net add" is called for an interface/net pair that
already exists, the error returned should be EEXIST, so the
user knows that the net is already configured.

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: Idab79ab288a11a2920793f27df235b4dfab497fe
Reviewed-on: https://review.whamcloud.com/35681
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>