nsproxy: use put_nsproxy() in switch_task_namespaces()

We already have a dedicated helper that handles reference count
checking so stop open-coding the reference count check in
switch_task_namespaces() and use the dedicated put_nsproxy() helper
instead.
Take the change to fix a whitespace issue too.
Signed-off-by: Hui Su <sh_def@163.com>
[christian.brauner@ubuntu.com: expand commit message]
Acked-by: NChristian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20201115180054.GA371317@rlkSigned-off-by: NChristian Brauner <christian.brauner@ubuntu.com>

kernel/nsproxy.c

@@ -250,8 +250,8 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
 	p->nsproxy = new;
 	task_unlock(p);
 
-	if (ns && atomic_dec_and_test(&ns->count))
-		free_nsproxy(ns);
+	if (ns)
+		put_nsproxy(ns);
 }
 
 void exit_task_namespaces(struct task_struct *p)