kernel/vm: Skip acquiring the available memory lock if there's nothing to unreserve.

This can happen when deleting VMCaches that were overcommitted or
never had any reservation, or caches that had their commitments
merged.

src/system/kernel/vm/vm.cpp

@@ -4697,10 +4697,11 @@ vm_kernel_address_space_left(void)
 void
 vm_unreserve_memory(size_t amount)
 {
+	if (amount == 0)
+		return;
+
 	mutex_lock(&sAvailableMemoryLock);
-
 	sAvailableMemory += amount;
-
 	mutex_unlock(&sAvailableMemoryLock);
 }