netfilter: ctnetlink: fix sleep in read-side lock section

Fix allocation with GFP_KERNEL in ctnetlink_create_conntrack() under
read-side lock sections.

This problem was introduced in 2.6.25.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>