neigh: reorder fields in struct neighbour

On 64bit arches, there are two 32bit holes that we can remove.

sizeof(struct neighbour) shrinks from 0xf8 to 0xf0 bytes

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>