tipc: don't directly overwrite node action_flags

Each node action flag should be set or cleared separately, instead
we now set the whole flags variable in one shot, and it's turned
out to be hard to see which other flags are affected. Therefore,
for instance, we explicitly clear TIPC_WAIT_OWN_LINKS_DOWN bit in
node_lost_contact().

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