netfilter: ctnetlink: fix GFP_KERNEL allocation under spinlock

The previous fix for the conntrack creation race (netfilter: ctnetlink:
fix conntrack creation race) missed a GFP_KERNEL allocation that is
now performed while holding a spinlock. Switch to GFP_ATOMIC.

Reported-and-tested-by: Zoltan Borbely <bozo@andrews.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>