nfs_opendir() created a context that held much more information than we
need for a readdir.  This patch introduces a slimmed-down
nfs_open_dir_context that contains only the cookie and the cred used for
RPC operations.  The new context will eventually be used to help detect
readdir loops.
Signed-off-by: NBryan Schumaker <bjschuma@netapp.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>