c/pb_planner: clamp calculated free space down to prevent overshoot

In the initial phase `health_report`` reported space may be incorrect as
not all partition storage information are included. Introduced clamping
down to actual disk free space + reclaimable size. This way we will never
go beyond the actual free space but when all reported values are up to date
the calculated free space should always be smaller then or equal to
free space plus reclaimable space.

Signed-off-by: Michal Maslanka <michal@redpanda.com>

src/v/cluster/partition_balancer_planner.cc

@@ -213,6 +213,16 @@ std::pair<uint64_t, uint64_t> partition_balancer_planner::get_node_bytes_info(
             }
         }
 
+        /**
+         * In the initial phase reported space may be incorrect as not all
+         * partition storage information are reported. Clamp down to actual
+         * disk free space + reclaimable size. This way we will never go beyond
+         * the actual free space but when all reported values are up to date
+         * the calculated free space should always be smaller then or equal to
+         * free space to reclaimable space.
+         */
+        total_free = std::min(
+          reclaimable_size + node_state.data_disk.free, total_free);
         // Clamp to total available target size.
         total_free = std::min(target_size, total_free);
         return std::make_pair(target_size, total_free);