qcow2: Round QCowL2Meta.offset down to cluster boundary

The offset within the cluster is already present as n_start and this is
what the code uses. QCowL2Meta.offset is only needed at a cluster
granularity.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -631,7 +631,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
     old_cluster = g_malloc(m->nb_clusters * sizeof(uint64_t));
 
     /* copy content of unmodified sectors */
-    start_sect = (m->offset & ~(s->cluster_size - 1)) >> 9;
+    start_sect = m->offset >> 9;
     if (m->n_start) {
         cow = true;
         qemu_co_mutex_unlock(&s->lock);
@@ -966,7 +966,7 @@ again:
                 .cluster_offset = keep_clusters == 0 ?
                                   alloc_cluster_offset : cluster_offset,
                 .alloc_offset   = alloc_cluster_offset,
-                .offset         = alloc_offset,
+                .offset         = alloc_offset & ~(s->cluster_size - 1),
                 .n_start        = keep_clusters == 0 ? n_start : 0,
                 .nb_clusters    = nb_clusters,
                 .nb_available   = MIN(requested_sectors, avail_sectors),

block/qcow2.h

@@ -199,12 +199,34 @@ struct QCowAIOCB;
 /* XXX This could be private for qcow2-cluster.c */
 typedef struct QCowL2Meta
 {
+    /** Guest offset of the first newly allocated cluster */
     uint64_t offset;
+
+    /** Host offset of the first cluster of the request */
     uint64_t cluster_offset;
+
+    /** Host offset of the first newly allocated cluster */
     uint64_t alloc_offset;
+
+    /**
+     * Number of sectors between the start of the first allocated cluster and
+     * the area that the guest actually writes to.
+     */
     int n_start;
+
+    /**
+     * Number of sectors from the start of the first allocated cluster to
+     * the end of the (possibly shortened) request
+     */
     int nb_available;
+
+    /** Number of newly allocated clusters */
     int nb_clusters;
+
+    /**
+     * Requests that overlap with this allocation and wait to be restarted
+     * when the allocating request has completed.
+     */
     CoQueue dependent_requests;
 
     QLIST_ENTRY(QCowL2Meta) next_in_flight;