[NETFILTER]: ctnetlink: return EEXIST instead of EINVAL for existing nat'ed conntracks
ctnetlink must return EEXIST for existing nat'ed conntracks instead of
EINVAL. Only return EINVAL if we try to update a conntrack with NAT
handlings (that is not allowed).
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>