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>

tools/testing/selftests/net/lib.sh

@@ -167,26 +167,30 @@ cleanup_all_ns()
 # setup_ns local remote
 setup_ns()
 {
-	local ns=""
 	local ns_name=""
 	local ns_list=()
 	for ns_name in "$@"; do
+		# avoid conflicts with local var: internal error
+		if [ "${ns_name}" = "ns_name" ]; then
+			echo "Failed to setup namespace '${ns_name}': invalid name"
+			cleanup_ns "${ns_list[@]}"
+			exit $ksft_fail
+		fi
+
 		# Some test may setup/remove same netns multi times
 		if [ -z "${!ns_name}" ]; then
-			ns="${ns_name,,}-$(mktemp -u XXXXXX)"
-			eval "${ns_name}=${ns}"
+			eval "${ns_name}=${ns_name,,}-$(mktemp -u XXXXXX)"
 		else
-			ns="${!ns_name}"
-			cleanup_ns "$ns"
+			cleanup_ns "${!ns_name}"
 		fi
 
-		if ! ip netns add "$ns"; then
+		if ! ip netns add "${!ns_name}"; then
 			echo "Failed to create namespace $ns_name"
 			cleanup_ns "${ns_list[@]}"
 			return $ksft_skip
 		fi
-		ip -n "$ns" link set lo up
-		ns_list+=("$ns")
+		ip -n "${!ns_name}" link set lo up
+		ns_list+=("${!ns_name}")
 	done
 	NS_LIST+=("${ns_list[@]}")
 }