selftests: net: lib: avoid error removing empty netns name

If there is an error to create the first netns with 'setup_ns()',
'cleanup_ns()' will be called with an empty string as first parameter.

The consequences is that 'cleanup_ns()' will try to delete an invalid
netns, and wait 20 seconds if the netns list is empty.

Instead of just checking if the name is not empty, convert the string
separated by spaces to an array. Manipulating the array is cleaner, and
calling 'cleanup_ns()' with an empty array will be a no-op.

Fixes: 25ae948b ("selftests/net: add lib.sh")
Cc: stable@vger.kernel.org
Acked-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20240605-upstream-net-20240605-selftests-net-lib-fixes-v1-2-b3afadd368c9@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>