ipv6: reorder struct inet6_ifaddr to remove padding on 64 bit builds

reorder struct inet6_ifaddr to remove padding on 64 bit builds
    
remove 8 bytes of padding so inet6_ifaddr becomes 192 bytes & fits into
a smaller slab.
    
Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>