netfilter: ctnetlink: group errors into logical errno sets

This patch groups ctnetlink errors into three logical sets:

* Malformed messages: if ctnetlink receives a message without some mandatory
attribute, then it returns EINVAL.
* Unsupported operations: if userspace tries to perform an unsupported
operation, then it returns EOPNOTSUPP.
* Unchangeable: if userspace tries to change some attribute of the
conntrack object that can only be set once, then it returns EBUSY.

This patch reduces the number of -EINVAL from 23 to 14 and it results in
5 -EBUSY and 6 -EOPNOTSUPP.

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>