netfilter: ctnetlink: allow changing NAT sequence adjustment in creation

This patch fixes an inconsistency in the current ctnetlink code
since NAT sequence adjustment bit can only be updated but not set
in the conntrack entry creation.

This patch is used by conntrackd to successfully recover newly
created entries that represent connections with helpers and NAT
payload mangling.

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>