[INET]: Consolidate xxx_the secret_rebuild

This code works with the generic data types as well, so
move this into inet_fragment.c

This move makes it possible to hide the secret_timer
management and the secret_rebuild routine completely in
the inet_fragment.c

Introduce the ->hashfn() callback in inet_frags() to get
the hashfun for a given inet_frag_queue() object.

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

include/net/inet_frag.h

@@ -36,6 +36,8 @@ struct inet_frags {
 	atomic_t		mem;
 	struct timer_list	secret_timer;
 	struct inet_frags_ctl	*ctl;
+
+	unsigned int		(*hashfn)(struct inet_frag_queue *);
 };
 
 void inet_frags_init(struct inet_frags *);