build failed in Linux 3.13

build failed in Linux 3.13 because inet6_hash_frag of net/ipv6/reassembly.c has been static since linux 3.13.
```
$ make
make -C /lib/modules/3.13-1-amd64/build M=$PWD;
make[1]: Entering directory `/usr/src/linux-headers-3.13-1-amd64'
  CC [M]  /home/tadokoro/src/NAT64/mod/fragment_db.o
/home/tadokoro/src/NAT64/mod/fragment_db.c: In function 'hash_function':
/home/tadokoro/src/NAT64/mod/fragment_db.c:156:3: error: implicit declaration of function 'inet6_hash_frag' [-Werror=implicit-function-declaration]
   result = inet6_hash_frag(key->ipv6.identification, &key->ipv6.src_addr,
   ^
cc1: some warnings being treated as errors
make[4]: *** [/home/tadokoro/src/NAT64/mod/fragment_db.o] Error 1
make[3]: *** [_module_/home/tadokoro/src/NAT64/mod] Error 2
make[2]: *** [sub-make] Error 2
make[1]: *** [all] Error 2
make[1]: Leaving directory `/usr/src/linux-headers-3.13-1-amd64'
make: *** [all] Error 2
```

I copied the code from reassembly.c.