Since commit 3c682146, iplink requires assigning negative
ifindex (-1) to the kernel when creating interface without
specifying index.

v2: checking whether index is -1, suggested by Cong Wang.

Cc: Cong Wang <cwang@twopensource.com>
Signed-off-by: Atzm Watanabe <atzm@stratosphere.co.jp>
Acked-by: Cong Wang <cwang@twopensource.com>