bql: Fix POSDIFF() to integer overflow aware.

POSDIFF() fails to take into account integer overflow case.

Signed-off-by: Hiroaki SHIMODA <shimoda.hiroaki@gmail.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Denys Fedoryshchenko <denys@visp.net.lb>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>