selftests/namespace: second threaded active reference count test

selftests/namespace: second threaded active reference count test

Test that a namespace remains active while a thread holds an fd to it.
Even after the thread exits, the namespace should remain active as long
as another thread holds a file descriptor to it.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-68-2e6f823ebdc0@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>