selftests/namespaces: second inactive namespace resurrection test

selftests/namespaces: second inactive namespace resurrection test

Test that socket file descriptors keep network namespaces active. Create
a network namespace, create a socket in it, then exit the namespace. The
namespace should remain active while the socket FD is held.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-56-2e6f823ebdc0@kernel.org
Tested-by: syzbot@syzkaller.appspotmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>