[IPV4/IPV6]: Fail registration if inet device construction fails

Now that netdev notifications can fail, we can use this to signal
errors during registration for IPv4/IPv6.  In particular, if we
fail to allocate memory for the inet device, we can fail the netdev
registration.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>