From: Eric Dumazet <redacted>
Date: Thu, 29 Nov 2012 09:43:24 -0800

Use a schem with a hash table of 256 (or 1024) slots.

Each slot/bucket has : 
  - Its own spinlock.
  - List of items
  - A limit of 5 (or so) elems in the list.

No more LRU, no more rehash (thanks to jhash and the random seed at boot
or first frag created), no more reader-writer lock.

Use a percpu_counter to implement ipfrag_low_thresh/ipfrag_high_thresh

If we limit the chain sizes to 5 elements, there is no need for
any thresholds at all.