vfs: push dentry_unhash on rmdir into file systems

Only a few file systems need this.  Start by pushing it down into each
fs rmdir method (except gfs2 and xfs) so it can be dealt with on a per-fs
basis.

This does not change behavior for any in-tree file systems.

Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/afs/dir.c

@@ -845,6 +845,8 @@ static int afs_rmdir(struct inode *dir, struct dentry *dentry)
 	_enter("{%x:%u},{%s}",
 	       dvnode->fid.vid, dvnode->fid.vnode, dentry->d_name.name);
 
+	dentry_unhash(dentry);
+
 	ret = -ENAMETOOLONG;
 	if (dentry->d_name.len >= AFSNAMEMAX)
 		goto error;