vhost-scsi: don't open-code kvfree

Now that we have kvfree, use it in vhost-scsi instead of
the open-coded version.

Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --cc drivers/vhost/scsi.c
Simple merge