[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>