pid: Introduce helper task_is_in_init_pid_ns()

Currently the kernel uses open code in multiple places to check if a
task is in the root PID namespace with the kind of format:

  if (task_active_pid_ns(current) == &init_pid_ns)
      do_something();

This patch creates a new helper function, task_is_in_init_pid_ns(), it
returns true if a passed task is in the root PID namespace, otherwise
returns false.  So it will be used to replace open codes.
Suggested-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: NLeo Yan <leo.yan@linaro.org>
Reviewed-by: NLeon Romanovsky <leonro@nvidia.com>
Acked-by: NSuzuki K Poulose <suzuki.poulose@arm.com>
Acked-by: NBalbir Singh <bsingharora@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>

include/linux/pid_namespace.h

@@ -86,4 +86,9 @@ extern struct pid_namespace *task_active_pid_ns(struct task_struct *tsk);
 void pidhash_init(void);
 void pid_idr_init(void);
 
+static inline bool task_is_in_init_pid_ns(struct task_struct *tsk)
+{
+	return task_active_pid_ns(tsk) == &init_pid_ns;
+}
+
 #endif /* _LINUX_PID_NS_H */