qcow2: check for NULL l2meta

In the case of a metadata preallocation with a large cluster size,
qcow2_alloc_cluster_offset() can allocate nothing and returns a
NULL l2meta. This patch checks for it and link2 l2 with only valid
l2meta.

Replace 9 and 512 with BDRV_SECTOR_BITS, BDRV_SECTOR_SIZE
respectively while at the function.
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -1404,34 +1404,34 @@ static int preallocate(BlockDriverState *bs)
     int ret;
     QCowL2Meta *meta;
 
-    nb_sectors = bdrv_getlength(bs) >> 9;
+    nb_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
     offset = 0;
 
     while (nb_sectors) {
-        num = MIN(nb_sectors, INT_MAX >> 9);
+        num = MIN(nb_sectors, INT_MAX >> BDRV_SECTOR_BITS);
         ret = qcow2_alloc_cluster_offset(bs, offset, &num,
                                          &host_offset, &meta);
         if (ret < 0) {
             return ret;
         }
 
-        ret = qcow2_alloc_cluster_link_l2(bs, meta);
-        if (ret < 0) {
-            qcow2_free_any_clusters(bs, meta->alloc_offset, meta->nb_clusters,
-                                    QCOW2_DISCARD_NEVER);
-            return ret;
-        }
-
-        /* There are no dependent requests, but we need to remove our request
-         * from the list of in-flight requests */
         if (meta != NULL) {
+            ret = qcow2_alloc_cluster_link_l2(bs, meta);
+            if (ret < 0) {
+                qcow2_free_any_clusters(bs, meta->alloc_offset,
+                                        meta->nb_clusters, QCOW2_DISCARD_NEVER);
+                return ret;
+            }
+
+            /* There are no dependent requests, but we need to remove our
+             * request from the list of in-flight requests */
             QLIST_REMOVE(meta, next_in_flight);
         }
 
         /* TODO Preallocate data if requested */
 
         nb_sectors -= num;
-        offset += num << 9;
+        offset += num << BDRV_SECTOR_BITS;
     }
 
     /*
@@ -1440,9 +1440,10 @@ static int preallocate(BlockDriverState *bs)
      * EOF). Extend the image to the last allocated sector.
      */
     if (host_offset != 0) {
-        uint8_t buf[512];
-        memset(buf, 0, 512);
-        ret = bdrv_write(bs->file, (host_offset >> 9) + num - 1, buf, 1);
+        uint8_t buf[BDRV_SECTOR_SIZE];
+        memset(buf, 0, BDRV_SECTOR_SIZE);
+        ret = bdrv_write(bs->file, (host_offset >> BDRV_SECTOR_BITS) + num - 1,
+                         buf, 1);
         if (ret < 0) {
             return ret;
         }