fs: dcache scale dentry refcount

Make d_count non-atomic and protect it with d_lock. This allows us to ensure a
0 refcount dentry remains 0 without dcache_lock. It is also fairly natural when
we start protecting many other dentry members with d_lock.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

fs/nfs/unlink.c

@@ -496,7 +496,7 @@ nfs_sillyrename(struct inode *dir, struct dentry *dentry)
 
 	dfprintk(VFS, "NFS: silly-rename(%s/%s, ct=%d)\n",
 		dentry->d_parent->d_name.name, dentry->d_name.name,
-		atomic_read(&dentry->d_count));
+		dentry->d_count);
 	nfs_inc_stats(dir, NFSIOS_SILLYRENAME);
 
 	/*