This cleans up the dcache code to always use the proper dcache functions
(d_unhashed and __d_drop) instead of accessing the dentry lists
directly.

In other words: use "d_unhashed(dentry)" instead of doing a manual
"list_empty(&dentry->d_hash)" test.  And use "__d_drop(dentry)" instead
of doing "list_del_init(&dentry->d_hash)" by hand.

This will help the dcache-rcu patches.