Ensure that we initialise desc->cache_entry_index correctly in
uncached_readdir().

Fixes: d1bacf9e ("NFS: add readdir cache array")
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: NAnna Schumaker <Anna.Schumaker@Netapp.com>