hh_lock was converted from rwlock to seqlock by Stephen.

To have a 100% benefit of this change, I suggest to place read mostly fields
of hh_cache in a separate cache line, because hh_refcnt may be changed quite
frequently on some busy machines.
Signed-off-by: NEric Dumazet <dada1@cosmosbay.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>