block: Minimize raw use of bds->total_sectors

bdrv_is_allocated_above() was relying on intermediate->total_sectors,
which is a field that can have stale contents depending on the value
of intermediate->has_variable_length.  An audit shows that we are safe
(we were first calling through bdrv_co_get_block_status() which in
turn calls bdrv_nb_sectors() and therefore just refreshed the current
length), but it's nicer to favor our accessor functions to avoid having
to repeat such an audit, even if it means refresh_total_sectors() is
called more frequently.
Suggested-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NManos Pitsidianakis <el13635@mail.ntua.gr>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/io.c

@@ -1947,6 +1947,7 @@ int bdrv_is_allocated_above(BlockDriverState *top,
     intermediate = top;
     while (intermediate && intermediate != base) {
         int64_t pnum_inter;
+        int64_t size_inter;
         int psectors_inter;
 
         ret = bdrv_is_allocated(intermediate, sector_num * BDRV_SECTOR_SIZE,
@@ -1962,15 +1963,12 @@ int bdrv_is_allocated_above(BlockDriverState *top,
             return 1;
         }
 
-        /*
-         * [sector_num, nb_sectors] is unallocated on top but intermediate
-         * might have
-         *
-         * [sector_num+x, nr_sectors] allocated.
-         */
+        size_inter = bdrv_nb_sectors(intermediate);
+        if (size_inter < 0) {
+            return size_inter;
+        }
         if (n > psectors_inter &&
-            (intermediate == top ||
-             sector_num + psectors_inter < intermediate->total_sectors)) {
+            (intermediate == top || sector_num + psectors_inter < size_inter)) {
             n = psectors_inter;
         }