[IPV6]: remove ifdef in route6 for xfrm6

The following patch create the usual static inline functions to disable
the xfrm6_init and xfrm6_fini function when XFRM is off.
That's allow to remove some ifdef and make the code a little more clear.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>