This fixes a panic doing the first READDIR or READDIRPLUS call when:

* the client is ia64 or any platform that actually implements
    flush_dcache_page(), and

  * the server returns fsinfo.dtpref >= client's PAGE_SIZE, and

  * the server does *not* return post-op attributes for the directory
    in the READDIR reply.

Problem diagnosed by Greg Banks <gnb@melbourne.sgi.com>

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>