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/hostfs/hostfs_kern.c

@@ -683,6 +683,8 @@ int hostfs_rmdir(struct inode *ino, struct dentry *dentry)
 	char *file;
 	int err;
 
+	dentry_unhash(dentry);
+
 	if ((file = dentry_name(dentry)) == NULL)
 		return -ENOMEM;
 	err = do_rmdir(file);