[INET]: Omit double hash calculations in xxx_frag_intern

Since the hash value is already calculated in xxx_find, we can
simply use it later. This is already done in netfilter code,
so make the same in ipv4 and ipv6.

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