vhost: vringh: Fix copy_to_iter return value check

vhost: vringh: Fix copy_to_iter return value check

The return value of copy_to_iter can't be negative, check whether the
copied length is equal to the requested length instead of checking for
negative values.

Cc: zhang jiao <zhangjiao2@cmss.chinamobile.com>
Link: https://lore.kernel.org/all/20250910091739.2999-1-zhangjiao2@cmss.chinamobile.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Fixes: 309bba39c945 ("vringh: iterate on iotlb_translate to handle large translations")
Link: https://patch.msgid.link/cd637504a6e3967954a9e80fc1b75e8c0978087b.1758723310.git.mst@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>