qcow2: Fix error handling in l2_allocate

l2_allocate has some intermediate states in which the image is inconsistent.
Change the order to write to the L1 table only after the new L2 table has
successfully been initialized.

Also reset the L2 cache in failure case, it's very likely wrong.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit 175e1152)

Conflicts:

	block/qcow2-cluster.c
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -231,13 +231,6 @@ static uint64_t *l2_allocate(BlockDriverState *bs, int l1_index)
         return NULL;
     }
 
-    /* update the L1 entry */
-
-    s->l1_table[l1_index] = l2_offset | QCOW_OFLAG_COPIED;
-    if (write_l1_entry(s, l1_index) < 0) {
-        return NULL;
-    }
-
     /* allocate a new entry in the l2 cache */
 
     min_index = l2_cache_new_entry(bs);
@@ -251,13 +244,19 @@ static uint64_t *l2_allocate(BlockDriverState *bs, int l1_index)
         if (bdrv_pread(s->hd, old_l2_offset,
                        l2_table, s->l2_size * sizeof(uint64_t)) !=
             s->l2_size * sizeof(uint64_t))
-            return NULL;
+            goto fail;
     }
     /* write the l2 table to the file */
     if (bdrv_pwrite(s->hd, l2_offset,
                     l2_table, s->l2_size * sizeof(uint64_t)) !=
         s->l2_size * sizeof(uint64_t))
-        return NULL;
+        goto fail;
+
+    /* update the L1 entry */
+    s->l1_table[l1_index] = l2_offset | QCOW_OFLAG_COPIED;
+    if (write_l1_entry(s, l1_index) < 0) {
+        goto fail;
+    }
 
     /* update the l2 cache entry */
 
@@ -265,6 +264,10 @@ static uint64_t *l2_allocate(BlockDriverState *bs, int l1_index)
     s->l2_cache_counts[min_index] = 1;
 
     return l2_table;
+
+fail:
+    qcow2_l2_cache_reset(bs);
+    return NULL;
 }
 
 static int count_contiguous_clusters(uint64_t nb_clusters, int cluster_size,