netfilter: nf_conntrack: add allocation flag to nf_conntrack_alloc

ctnetlink does not need to allocate the conntrack entries with GFP_ATOMIC
as its code is executed in user context.

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>