repair: Consider memory bloat when calculate repair parallelism

The repair parallelism is calculated by the number of memory allocated to
repair and memory usage per repair instance. Currently, it does not
consider memory bloat issues (e.g., issue #8640) which cause repair to
use more memory and cause std::bad_alloc.

Be more conservative when calculating the parallelism to avoid repair
using too much memory.

Fixes #8641

Closes #8652

repair/repair.cc

@@ -354,7 +354,7 @@ float node_ops_metrics::repair_finished_percentage() {
 tracker::tracker(size_t nr_shards, size_t max_repair_memory)
     : _shutdown(false)
     , _repairs(nr_shards) {
-    auto nr = std::max(size_t(1), size_t(max_repair_memory / max_repair_memory_per_range()));
+    auto nr = std::max(size_t(1), size_t(max_repair_memory / max_repair_memory_per_range() / 4));
     rlogger.info("Setting max_repair_memory={}, max_repair_memory_per_range={}, max_repair_ranges_in_parallel={}",
         max_repair_memory, max_repair_memory_per_range(), nr);
     _range_parallelism_semaphores.reserve(nr_shards);