xfrm: expose local_rxpmtu via ipv6_stubs

We cannot call this function from the core kernel unless we would force
CONFIG_IPV6=y.

Therefore expose this via ipv6_stubs so we can call it from net/xfrm
in the followup patch.

Since the call is expected to be unlikely, no extra code for the IPV6=y
case is added and we will always eat the indirection cost.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>