node_config: Enable large alloc warnings above 128KiB by default

We already have the reactor stall warnings on in "prod" and arguably the
large allocs are more important as they actually might result in a
crash.

Further the large alloc threshold automatically increases after each
warning so it's unlikely for this to be very spamy.

src/v/config/node_config.cc

@@ -158,7 +158,7 @@ node_config::node_config() noexcept
       "memory_allocation_warning_threshold",
       "Enables log messages for allocations greater than the given size.",
       {.visibility = visibility::tunable},
-      std::nullopt)
+      128_KiB + 1) // 128 KiB is the largest allowed allocation size
   , storage_failure_injection_enabled(
       *this,
       "storage_failure_injection_enabled",