generic: show locks info owned by dead process

generic: show locks info owned by dead process

Regression test case for kernel patchset:
    fs/lock: show locks info owned by dead/invisible processes

Open new fd by exec shell built-in, then require exclusive lock
by flock(1) command. Checking /proc/locks for the lock info.

This patchset post by Konstantin Khorenko <khorenko@virtuozzo.com>
has not been merged v4.18-rc8.

[Eryu: require $FLOCK_PROG and add more comments]

Signed-off-by: Xiong Murphy Zhou <xzhou@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>