xfrm: Fix the gc threshold value for ipv4

The xfrm gc threshold value depends on ip_rt_max_size. This
value was set to INT_MAX with the routing cache removal patch,
so we start doing garbage collecting when we have INT_MAX/2
IPsec routes cached. Fix this by going back to the static
threshold of 1024 routes.

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