migration/block: replace uses of blk_nb_sectors that do not check result

Uses of blk_nb_sectors must check whether the result is negative.
Otherwise, underflow can happen.  Fortunately, alloc_aio_bitmap()
and bmds_aio_inflight() both have an alternative way to retrieve the
number of sectors in the file.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20230407153303.391121-6-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

migration/block.c

@@ -195,7 +195,7 @@ static int bmds_aio_inflight(BlkMigDevState *bmds, int64_t sector)
 {
     int64_t chunk = sector / (int64_t)BDRV_SECTORS_PER_DIRTY_CHUNK;
 
-    if (sector < blk_nb_sectors(bmds->blk)) {
+    if (sector < bmds->total_sectors) {
         return !!(bmds->aio_bitmap[chunk / (sizeof(unsigned long) * 8)] &
             (1UL << (chunk % (sizeof(unsigned long) * 8))));
     } else {
@@ -229,10 +229,9 @@ static void bmds_set_aio_inflight(BlkMigDevState *bmds, int64_t sector_num,
 
 static void alloc_aio_bitmap(BlkMigDevState *bmds)
 {
-    BlockBackend *bb = bmds->blk;
     int64_t bitmap_size;
 
-    bitmap_size = blk_nb_sectors(bb) + BDRV_SECTORS_PER_DIRTY_CHUNK * 8 - 1;
+    bitmap_size = bmds->total_sectors + BDRV_SECTORS_PER_DIRTY_CHUNK * 8 - 1;
     bitmap_size /= BDRV_SECTORS_PER_DIRTY_CHUNK * 8;
 
     bmds->aio_bitmap = g_malloc0(bitmap_size);