net: neigh: use long type to store jiffies delta

A difference of two unsigned long needs long storage.

Fixes: c7fb64db

 ("[NETLINK]: Neighbour table configuration and statistics via rtnetlink")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>