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: Suzuki K Poulose <[email protected]>
Signed-off-by: Leo Yan <[email protected]>
Reviewed-by: Leon Romanovsky <[email protected]>
Acked-by: Suzuki K Poulose <[email protected]>
Acked-by: Balbir Singh <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Leo Yan

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 */