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

This patch changes the way DCACHE_REFERENCED flag is used. It
got messed up in dcache_rcu iterations. I hope this will be ok now.

The flag was meant to be advisory flag which is used while
prune_dcache() so as not to free dentries which have recently
entered d_lru list. At first pass in prune_dcache the dentries
marked DCACHE_REFERENCED are left with the flag reset. and they
are freed in the next pass.

So, now we mark the dentry as DCACHE_REFERENCED when it is first
entering the d_lru list in dput() and resetthe flag in prune_dcache().
If the flag remains reset in the next call to prune_dcache(), the
dentry is then freed.

Also I don't think any file system have to use this flag as it is taken
care by the dcache layer. The patch removes such code from a few of file
systems. Moreover these filesystems were anyway doing worng thing as they
were changing the flag out of dcache_lock.

Changes:
o dput() marks dentry DCACHE_REFERENCED when it is added to the dentry_unused
  list
o no need to set the flag in dget, dget_locked, d_lookup as these guys anyway
  increments the ref count.
o check the ref count in prune_dcache and use DCACHE_REFERENCED flag just for
  two stage aging.
o remove code for setting DACACHE_REFERENCED from reiserfs, fat, xfs and
  exportfs.