net: Remove unused parameter of xfrm_gen_index()

In commit 2518c7c2 ("[XFRM]: Hash
policies when non-prefixed."), the last use of xfrm_gen_policy() first
argument was removed, but the argument was left behind in the
prototype.

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>