fs/lock: show locks taken by processes from another pidns

Currently if we face a lock taken by a process invisible in the current
pidns we skip the lock completely, but this

1) makes the output not that nice
    (root@vz7)/: cat /proc/${PID_A2}/fdinfo/3
    pos:    4
    flags:  02100002
    mnt_id: 257
    lock:   (root@vz7)/:

2) makes it more difficult to debug issues with leaked flocks
   if you get error on lock, but don't see any locks in /proc/$id/fdinfo/$file

Let's show information about such locks again as previously, but
show zero in the owner pid field.

After the patch:
===============
(root@vz7)/:cat /proc/${PID_A2}/fdinfo/3
pos:    4
flags:  02100002
mnt_id: 295
lock:   1: FLOCK  ADVISORY  WRITE 0 b6:f8a61:529946 0 EOF

Fixes: 9d5b86ac13 ("fs/locks: Remove fl_nspid and use fs-specific l_pid for remote locks")
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
Acked-by: Andrey Vagin <avagin@openvz.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>

fs/locks.c

@@ -2633,12 +2633,10 @@ static void lock_get_status(struct seq_file *f, struct file_lock *fl,
 
 	fl_pid = locks_translate_pid(fl, proc_pidns);
 	/*
-	 * If there isn't a fl_pid don't display who is waiting on
-	 * the lock if we are called from locks_show, or if we are
-	 * called from __show_fd_info - skip lock entirely
+	 * If lock owner is dead (and pid is freed) or not visible in current
+	 * pidns, zero is shown as a pid value. Check lock info from
+	 * init_pid_ns to get saved lock pid value.
 	 */
-	if (fl_pid == 0)
-		return;
 
 	if (fl->fl_file != NULL)
 		inode = locks_inode(fl->fl_file);