netfilter: ctnetlink: sleepable allocation with spin lock bh

This patch removes a GFP_KERNEL allocation while holding a spin lock with
bottom halves disabled in ctnetlink_change_helper().

This problem was introduced in 2.6.23 with the netfilter extension
infrastructure.

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>