rtnetlink: fix a memory leak when ->newlink fails

rtnetlink: fix a memory leak when ->newlink fails

It is possible that ->newlink() fails before registering
the device, in this case we should just free it, it's
safe to call free_netdev().

Fixes: commit aa8f34a1a6e1baa9703 (net: correct error path in rtnl_newlink())
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>