[IPV4] devinet: Register inetdev earlier.

This patch allocates inetdev at registration for all devices
in line with IPv6.  This allows sysctl configuration on the
devices to occur before they're brought up or addresses are
added.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>