From: Maneesh Soni <maneesh@in.ibm.com>

Here is a patch to use seqlock for real_lookup race with d_lookup as suggested
by Linus. The race condition can result in duplicate dentry when d_lookup
fails due concurrent d_move in some unrelated directory.

Apart from real_lookup, lookup_hash()->cached_lookup() can also fail due
to same reason. So, for that I am doing the d_lookup again.

Now we have __d_lookup (called from do_lookup() during pathwalk) and
d_lookup which uses seqlock to protect againt rename race.

dcachebench numbers (lower is better) don't have much difference on a 4-way
PIII xeon SMP box.

base-2565
Average usec/iteration  19059.4
Standard Deviation      503.07

base-2565 + seq_lock
Average usec/iteration  18843.2
Standard Deviation      450.57