zloop: fix write pointer calculation in zloop_forget_cache

zloop: fix write pointer calculation in zloop_forget_cache

The write pointer is absolute and in sector units, so we need to
convert it to a relative byte address first.

Fixes: c505448748f7 ("zloop: forget write cache on force removal")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://patch.msgid.link/20260414081811.549755-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>