rtnetlink: potential ERR_PTR dereference

In the original code, if rtnl_create_link() returned an ERR_PTR then that
would get passed to rtnl_configure_link() which dereferences it.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>