[PATCH] : struct dentry : place d_hash close to d_parent and d_name to speedup lookups

[Eric Dumazet <dada1@cosmosbay.com>]

dentry cache uses sophisticated RCU technology (and prefetching if available) 
but touches 2 cache lines per dentry during hlist lookup.

This patch moves d_hash in the same cache line than d_parent and d_name fields 
so that :

1) One cache line is needed instead of two.
2) the hlist_for_each_rcu() prefetching has a chance to bring all the needed 
data in advance, not only the part that includes d_hash.next.

I also changed one old comment that was wrong for 64bits.

A further optimisation would be to separate dentry in two parts, one that is 
mostly read, and one writen (d_count/d_lock) to avoid false sharing on 
SMP/NUMA but this would need different field placement depending on 32bits or 
64bits platform.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>