dma-helpers: avoid calling dma_bdrv_unmap() twice

Calling dma_bdrv_unmap() twice is not necessary and may cause
potential problems if some code changes.

Signed-off-by: Jules Wang <junqing.wang@cs2c.com.cn>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

dma-helpers.c

@@ -143,12 +143,12 @@ static void dma_bdrv_cb(void *opaque, int ret)
 
     dbs->acb = NULL;
     dbs->sector_num += dbs->iov.size / 512;
-    dma_bdrv_unmap(dbs);
 
     if (dbs->sg_cur_index == dbs->sg->nsg || ret < 0) {
         dma_complete(dbs, ret);
         return;
     }
+    dma_bdrv_unmap(dbs);
 
     while (dbs->sg_cur_index < dbs->sg->nsg) {
         cur_addr = dbs->sg->sg[dbs->sg_cur_index].base + dbs->sg_cur_byte;