staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5875
Reviewed-on: http://review.whamcloud.com/13056


Reviewed-by: Isaac Huang <he.huang@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>