xfrm: Use a static gc threshold value for ipv6

Unlike ipv4 did, ipv6 does not handle the maximum number of cached
routes dynamically. So no need to try to handle the IPsec gc threshold
value dynamically. This patch sets the IPsec gc threshold value back to
1024 routes, as it is for non-IPsec routes.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>