rpc/connection_cache: always acquire mutex before accessing shared state

Co-authored-by: Stephan Dollberg <stephan@redpanda.com>

src/v/rpc/connection_cache.cc

@@ -196,8 +196,6 @@ ss::future<> connection_cache::stop() {
 ss::future<> connection_cache::apply_changes(
   connection_allocation_strategy::changes changes,
   std::optional<connection_config> config) {
-    auto units = co_await _coordinator_state->mtx.get_units();
-
     // Add connections we have the config for.
     if (config) {
         for (auto& con_add : changes.add_connections) {
@@ -250,7 +248,9 @@ ss::future<> connection_cache::remove_broker_client_coordinator(
     }
     auto holder = _gate.hold();
 
+    auto units = co_await _coordinator_state->mtx.get_units();
     auto& alloc_strat = _coordinator_state->alloc_strat;
+
     if (alloc_strat.has_connection_assignments_for(dest)) {
         auto changes = alloc_strat.remove_connection_assignments_for(dest);
         co_await apply_changes(changes, std::nullopt);
@@ -279,6 +279,7 @@ connection_cache::update_broker_client_coordinator(connection_config cfg) {
     }
     auto holder = _gate.hold();
 
+    auto units = co_await _coordinator_state->mtx.get_units();
     auto& alloc_strat = _coordinator_state->alloc_strat;
 
     if (!alloc_strat.has_connection_assignments_for(cfg.dest_node)) {