NFS: Don't skip directory entries when doing uncached readdir

[ Upstream commit ce292d8f ]

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

Fixes: d1bacf9e

 ("NFS: add readdir cache array")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>