pids: sys_setpgid: use change_pid() helper

Use change_pid() instead of detach_pid() + attach_pid() in sys_setpgid().

This way task_pgrp() is not NULL in between.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc:  "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sys.c

@@ -978,8 +978,7 @@ asmlinkage long sys_setpgid(pid_t pid, pid_t pgid)
 		goto out;
 
 	if (task_pgrp(p) != pgrp) {
-		detach_pid(p, PIDTYPE_PGID);
-		attach_pid(p, PIDTYPE_PGID, pgrp);
+		change_pid(p, PIDTYPE_PGID, pgrp);
 		set_task_pgrp(p, pid_nr(pgrp));
 	}