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/jffs2/dir.c

@@ -609,6 +609,8 @@ static int jffs2_rmdir (struct inode *dir_i, struct dentry *dentry)
 	int ret;
 	uint32_t now = get_seconds();
 
+	dentry_unhash(dentry);
+
 	for (fd = f->dents ; fd; fd = fd->next) {
 		if (fd->ino)
 			return -ENOTEMPTY;