[PATCH] NFSv4: add support for rdattr_error in NFSv4 readdir requests.

 Request RDATTR_ERROR as an attribute in readdir to distinguish between a
 directory being within an absent filesystem or one (or more) of its entries.

Signed-off-by: Manoj Naik <manoj@almaden.ibm.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>