selftests/net/lib: no need to record ns name if it already exist

There is no need to add the name to ns_list again if the netns already
recoreded.

Fixes: 25ae948b ("selftests/net: add lib.sh")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>