tcm_vhost: Add missed lock in vhost_scsi_clear_endpoint()

tv_tpg->tv_tpg_vhost_count should be protected by tv_tpg->tv_tpg_mutex.

Signed-off-by: Asias He <asias@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>