[NET]: Change "not found" return value for rule lookup

This changes the "not found" error return for the lookup
function to -ESRCH so that it can be distinguished from
the case where a rule or route resulting in -ENETUNREACH
has been found during the search.

It fixes a bug where if DECnet was compiled with routing
support, but no routes were added to the routing table,
it was failing to fall back to endnode routing.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Patrick Caulfield <pcaulfie@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>