udp: reorder udp_iter_state to remove padding on 64bit builds

reorder udp_iter_state to remove padding on 64bit builds

shrinks from 24 to 16 bytes, moving to a smaller slab when
CONFIG_NET_NS is undefined & seq_net_private = {}

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>