rtnl: simplify error return path in rtnl_create_link()

There is only one possible error path which reaches the err label, so
return ERR_PTR(-ENOMEM) directly if alloc_netdev_mqs() fails. This also
allows to omit the err variable.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>