generic: truncate heavily reflinked file

generic: truncate heavily reflinked file

In btrfs, there is a bug about btrfs' truncate codes, it'll leak
some fs space as the truncate operation proceeds. If this truncate
operation is very large, later metadata request in this truncate
operation may fail for enospc error. I also have sent a kernel
patch fot btrfs to fix this issue.

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>