[INET]: Consolidate the xxx_frag_destroy

To make in possible we need to know the exact frag queue
size for inet_frags->mem management and two callbacks:

 * to destoy the skb (optional, used in conntracks only)
 * to free the queue itself (mandatory, but later I plan to
   move the allocation and the destruction of frag_queues
   into the common place, so this callback will most likely
   be optional too).

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>