LU-5875 lnet: return -EEXIST if NI is not unique

LU-5875 lnet: return -EEXIST if NI is not unique

Return -EEXIST and not -EINVAL when trying to add a
network interface which is not unique.

Some minor cleanup in api-ni.c

Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: Ic74a768c0e7688ba0e35740e2ca2ac9ae4f999ea
Reviewed-on: http://review.whamcloud.com/13056
Tested-by: Jenkins
Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>