When scanning route cache hash table, we can avoid taking locks for empty 
buckets.

I'm not sure it's worth it in this case.  A rcu_read_lock() is a nop
(on non preemptible kernel) to very cheap (non atomic increment/decrement in 
cached task_struct) 

If you really wanted to make it faster I think a better strategy
would be to not take it every bucket, but maybe only every 100th
or so. That would still keep preempt off times reasonably low, but
even help when the table is filled. But not sure it would 
be even measurable, really it should be quite cheap.

-Andi