net: if: Reshuffle fields in net_if and related structs

This does not save any space but sets the fields in the
structs in more natural order. Move IPv6 related
fields in net_if into internal ipv6 struct so that all
IPv6 fields are located inside one struct. Same thing
is done for IPv4 fields which are now located inside
IPv4 internal struct in net_if.

There is no functionality changes by this commit.

Change-Id: I7d72ec0a28e2b88c79a4c294655d5ef6da6ccb25
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>