vhost: reduce stack usage in log_used

Fix the warning: [-Werror=-Wframe-larger-than=]

drivers/vhost/vhost.c: In function log_used:
drivers/vhost/vhost.c:1906:1:
warning: the frame size of 1040 bytes is larger than 1024 bytes

Signed-off-by: Li Wang <li.wang@windriver.com>
Link: https://lore.kernel.org/r/1600106889-25013-1-git-send-email-li.wang@windriver.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>