tipc: eliminate checking netns if node established

[ Upstream commit d408bef4bfa60bac665b6e7239269570039a968b ]

Currently, we scan over all network namespaces at each received
discovery message in order to check if the sending peer might be
present in a host local namespaces.

This is unnecessary since we can assume that a peer will not change its
location during an established session.

We now improve the condition for this testing so that we don't perform
any redundant scans.

Fixes: f73b12812a3d ("tipc: improve throughput between nodes in netns")
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: c244c092f1ed ("tipc: fix unexpected link reset due to discovery messages")
Signed-off-by: Sasha Levin <sashal@kernel.org>