reiserfs: fixup xattr_root caching

 The xattr_root caching was broken from my previous patch set. It wouldn't
 cause corruption, but could cause decreased performance due to allocating
 a larger chunk of the journal (~ 27 blocks) than it would actually use.

 This patch loads the xattr root dentry at xattr initialization and creates
 it on-demand. Since we're using the cached dentry, there's no point
 in keeping lookup_or_create_dir around, so that's removed.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>