netfilter: ctnetlink: cleanup conntrack update preliminary checkings

This patch moves the preliminary checkings that must be fulfilled
to update a conntrack, which are the following:

 * NAT manglings cannot be updated
 * Changing the master conntrack is not allowed.

This patch is a cleanup.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>