tipc: introduce message evaluation function

When a message arrives in a node and finds no destination
socket, we may need to drop it, reject it, or forward it after
a secondary destination lookup. The latter two cases currently
results in a code path that is perceived as complex, because it
follows a deep call chain via obscure functions such as
net_route_named_msg() and net_route_msg().

We now introduce a function, tipc_msg_eval(), that takes the
decision about whether such a message should be rejected or
forwarded, but leaves it to the caller to actually perform
the indicated action.

If the decision is 'reject', it is still the task of the recently
introduced function tipc_msg_reverse() to take the final decision
about whether the message is rejectable or not. In the latter case
it drops the message.

As a result of this change, we can finally eliminate the function
net_route_named_msg(), and hence become independent of net_route_msg().

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>