selftests: net: lib: remove 'ns' var in setup_ns

selftests: net: lib: remove 'ns' var in setup_ns

The helper setup_ns() doesn't work when a net namespace named "ns" is
passed to it.

For example, in net/mptcp/diag.sh, the name of the namespace is "ns". If
"setup_ns ns" is used in it, diag.sh fails with errors:

  Invalid netns name "./mptcp_connect"
  Cannot open network namespace "10000": No such file or directory
  Cannot open network namespace "10000": No such file or directory

That is because "ns" is also a local variable in setup_ns, and it will
not set the value for the global variable that has been giving in
argument. To solve this, we could rename the variable, but it sounds
better to drop it, as we can resolve the name using the variable passed
in argument instead.

The other local variables -- "ns_list" and "ns_name" -- are more
unlikely to conflict with existing global variables. They don't seem to
be currently used in any other net selftests.

Co-developed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240607-upstream-net-next-20240607-selftests-mptcp-net-lib-v1-4-e36986faac94@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>