staging/lustre: don't assert ln_refcount in LNetGetId

If LNetNIInit() fails, we'll get zero ln_refcount. So fail
LNetGetId() properly instead of asserting.

We can get to it when socklnd fails to scan network interfaces,
which is possible if Lustre is builtin.

Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>