[NETFILTER] ctnetlink: get_conntrack can use GFP_KERNEL

ctnetlink_get_conntrack is always called from user context, so GFP_KERNEL
is enough.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>