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: 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>