[IPV4]: Restore old behaviour of default config values

Previously inet devices were only constructed when addresses are added
(or rarely in ipmr).  Therefore the default config values they get are
the ones at the time of these operations.

Now that we're creating inet devices earlier, this changes the
behaviour of default config values in an incompatible way (see bug
#8519).

This patch creates a compromise by setting the default values at the
same point as before but only for those that have not been explicitly
set by the user since the inet device's creation.

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