fs/lock: skip lock owner pid translation in case we are in init_pid_ns

If the flock owner process is dead and its pid has been already freed,
pid translation won't work, but we still want to show flock owner pid
number when expecting /proc/$PID/fdinfo/$FD in init pidns.

Reproducer:
process A	process A1	process A2
fork()--------->
exit()		open()
		flock()
		fork()--------->
		exit()		sleep()

Before the patch:
================
(root@vz7)/: cat /proc/${PID_A2}/fdinfo/3
pos:    4
flags:  02100002
mnt_id: 257
lock:   (root@vz7)/:

After the patch:
===============
(root@vz7)/:cat /proc/${PID_A2}/fdinfo/3
pos:    4
flags:  02100002
mnt_id: 295
lock:   1: FLOCK  ADVISORY  WRITE ${PID_A1} b6:f8a61:529946 0 EOF

Fixes: 9d5b86ac ("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

@@ -2072,6 +2072,13 @@ static pid_t locks_translate_pid(struct file_lock *fl, struct pid_namespace *ns)
 		return -1;
 	if (IS_REMOTELCK(fl))
 		return fl->fl_pid;
+	/*
+	 * If the flock owner process is dead and its pid has been already
+	 * freed, the translation below won't work, but we still want to show
+	 * flock owner pid number in init pidns.
+	 */
+	if (ns == &init_pid_ns)
+		return (pid_t)fl->fl_pid;
 
 	rcu_read_lock();
 	pid = find_pid_ns(fl->fl_pid, &init_pid_ns);