[IPV4]: Only panic if inetdev_init fails for loopback

When I made the inetdev_init call work on all devices I incorrectly
left in the panic call as well.  It is obviously undesirable to
panic on an allocation failure for a normal network device.  This
patch moves the panic call under the loopback if clause.

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