[IPV6]: Uninline netfilter okfns

Uninline netfilter okfns for those cases where gcc can generate tail-calls.

Before:
   text    data     bss     dec     hex filename
8994153 1016524  524652 10535329         a0c1a1 vmlinux

After:
   text    data     bss     dec     hex filename
8992761 1016524  524652 10533937         a0bc31 vmlinux
-------------------------------------------------------
  -1392

All cases have been verified to generate tail-calls with and without netfilter.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>